The Capability Affordance Model: Comparing Medical

The Capability Affordance Model: Comparing Medical Capabilities Vaughan Michell Informatics Research Centre, Henley Business School, University of Reading, UK [email protected]

Abstract. Existing capability models lack qualitative and quantitative means to compare business capabilities. This paper extends previous work and uses affordance theories to consistently model and analyse capabilities. We use the concept of objective and subjective affordances to model capability as a tuple of a set of resource affordance system mechanisms and action paths, dependent on one or more critical affordance factors. We identify an affordance chain of subjective affordances by which affordances work together to enable an action and an affordance path that links action affordances to create a capability system. We define the mechanism and path underlying capability. We show how affordance modelling notation, AMN, can represent affordances comprising a capability. We propose a method to quantitatively and qualitatively compare capabilities using efficiency, effectiveness and quality metrics. The method is demonstrated by a medical example comparing the capability of syringe and needless anaesthetic systems. Keywords: Capability, Affordance, Active Resources, Driving Resources, Passive Resources, Affordance Mechanism, Affordance Path, Affordance Chain, Critical Affordance Factor, Affordance Efficiency, Affordance Effectiveness, Affordance Quality, Affordance system.

1

Introduction

The term ‘Capability’ is well used, but has widely varying definitions in the business community. It indicates ‘potential function or ability’ of a firms resources [1], the ‘ability to add value’ [2], ‘organisational ability’ [3], a ‘capacity to perform a task’ [4] among many others [5]. A reliable definition and a means to model and compare capabilities of business processes, departments and resources are increasingly required to support enterprise architecture models. However, analysis of capabilities has been mainly at high level [4][6][7][8][9] with models supporting widely varying definitions and no consistent approach to specify or quantify capabilities. In a previous paper [5] we analysed various capability definitions to develop a focused and specific business capability definition based on resource theory. We defined capability as ‘the potential of a business resource (or groups of resources) to produce customer value by acting on their environment via a process (P) using other tangible (Rt) and intangible Resources (Ri) [5] and produced a high level model to capture different capabilities. However, further work was still required to resolve gaps; B. Shishkov (Ed.): BMSD 2012, LNBIP 142, pp. 102–124, 2013. © Springer-Verlag Berlin Heidelberg 2013

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a) how capability behaviour and qualities could be modelled and analysed at a detailed level. A model b) of the mechanism of interaction between the resources that creates this ‘potential’ and the factors that affect capability is also required. This model should also enable c) a consistent quantitative and qualitative comparison of capabilities. This paper addresses these problems by adapting existing methods that consistently model resource interactions and the process by which they interact. Affordance models at the detailed level of objects interacting provide the basis for the mechanism we seek [20]. Affordance theory postulated Gibson [10] models the interaction properties of objects in the human environment. However, like capability, affordance theory suffers from a range of definitions and competing views. Our approach is to i) resolve the conflicting definitions of affordances, ii) show how affordance theory can be used to model resource interaction that underlies capability and iii) to show how affordance theory can be integrated with the capability model. Our aim is to provide a coherent measurable mechanism for defining and analysing business capability. 1.1

Objectives and Layout of the Paper

This paper proposes an integrated capability-affordance model that combines the high level capability model with a detailed qualitative and quantitative affordance model of resource interactions. This paper builds on our previous work on capability. It extends and develops affordance theory and provides a mechanism that links the competing views. It defines measurements of affordance that can be used to compare capabilities. It then integrates these methods into the findings of the earlier paper to develop a capability-affordance model that enables detailed analysis and modelling of business capabilities. Section 1. introduces definitions used and details of the high level capability model and the background to affordance theory. In section 2. affordance theory is analysed and the concept of objective and subjective affordance are developed to resolve the different views and a provide mechanism to model the resource interactions and resolve gap b). Section 3. proposes a method to analyse affordances to address gap a) and show how the capability and affordance models can be integrated to form a capability-affordance model. Section 4. shows how capabilities in the high level model relate to systems of affordances at the detailed level. In section 5. we integrate the high level capability with the developed theory of affordances to provide the capability- affordance model and a modelling notation resolving gap a). We use the concept of efficiency, effectiveness and quality to provide a quantitative method of comparing capabilities to resolve gap c). In section 6 we demonstrate the model in a 5 step method to a health related example to compare the capabilities of two different systems of resources with the same capability goal. Section 7 summarises and concludes the paper with further work. 1.2

Definitions

The Business Environment. A business environment E comprises a set of n objects we called business object resources Ri where i has value 1 to n. [11]. Environment E comprises a set of {resources Ri}. The set of object resources {Ri} have perceivable features p and facts f about the object eg size, weight, chemical composition etc [12].

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Agents and Resources. A human or artificially intelligent agent is a resource object that can perceive its own environment through sensors and acts on the environment according to their self-motivations through effectors. [11][13]. Agent resources (R) have a potential for interaction with the environment to create value. A driving resource refers to object resources actively enabling the business transformation through substantive actions, compared with those (objects) passively used in the transformation involved in a business process [5]. Driving resources in a business process are autonomous agents (human/artificial) controlling a range of transformations. Active resources (Ra) are capable of exerting a change of state on other object resources in a transformation (note driving resources must be active resources). Passive resources (Rp) require other agents to realise their capability ie they are inert and not capable of their own motion or change of state. Processes and Transformation 1.3

The High Level Capability Model

Our original paper used an analysis of capability definitions and the resource based view to develop a capability model. For any process in a business environment that meets a business goal, the tangible and intangible resources (eg people, materials, equipment, knowledge, documents) undergo transformations and a change of state as a result of an action of one resource interacting with another. The driven resources drive other resources via a defined series of process interactions to enable the transformation Each of these resources has a capability of adding benefit of a specific value dependent on what resource and process is used to execute the capability. Cv (capability of value V) = f (resource, process P) .

(1)

The Capability Cv results from transformation interactions between two or more resources that increase the business value [14]. of the transformed resources with respect to a business client). Hence Cv is f(resource interaction, process of interaction). Driving resource Potential Process P

process goals G

Capability uses

meets Input Resources Value V

Activity to transform resources

Output Resources (utilised/consumed) Value V’> V

Fig. 1. High Level Capability Model

Consider a surgical example; an anaesthetist (hospital resource) has a capability to anaesthetise a patient and deliver the value of inducing an immobile sleep state in a patient (a measure of V’). The anaesthetist acts as an active driving resource


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coordinating and using other active tangible resources (eg drug, patient) and passive resources (syringe, swab, ampoule) to achieve the goal of ensuring the patient is immobile and unconscious. He/she uses their intangible knowledge of anatomy and drug behaviour and a sequence of coordinated actions (process) to prepare the anaesthetic drug and administer it with a tool, a syringe, summarised in the capability specification table below. Table 1. High Level Capability Specification Table Capability f(P, Rt, Ri): to anaesthetise patient Driving Resource: Anaesthetist Goal: to ensure patient is immobile and unconscious Value: immobile sleep state induced in patient Process P 1 dilute drug 2 load drug identify vein and target 3 site 4 inject anaesthetic 5 control inspection 6 swab site

Tangible Resources (Rt)

Intangible Resources (Ri)

Anaesthetist, syringe, drug and ampdrug dilution knowledge Anaesthetist, syringe, drug and ampsyringe injection experience Anaesthetist

anatomy and physiology knowledge

Anaesthetist, syringe, diluted drug syringe injection experience Anaesthetist

syringe injection experience

Anaesthetist, swab

syringe injection experience

In summary; All object resources possess the capability to transform/be transformed. Capability refers to the potential transformation of a system of resources. Capability is a function of the object resource transformation mechanism and the process or ‘way’ the resources interact or are driven to interact in space and time. However, the high level capability model lacks a means to qualify and measure the resource interaction transformation mechanism and process in a repeatable and consistent way that enables us to measure and compare capabilities and make resource and business decisions, such as selection, optimised use and divestment of assets. 1.4

Affordance Theory

The theory of affordance [10] provides a perspective on the interaction of objects in an environment that supports the capability model. Gibson’s definition of affordances as ‘an invariant combination of properties of substance and surface taken with reference to an animal’ identifies the idea of affordances as subjective properties of the agent- object combination. Ortman and Kuhn [12] noted that affordances are ‘qualities of entities in an environment’ that includes potential for action. Affordances also ‘capture the essential world and object properties, in terms of the actions the agent is able to perform’ [15]. Heft [16] highlights the importance of the subjective perspective by identifying that how agents perceive an affordance potential of an object can change with use experience and need. Maier in his work on design affordances notes that such artefact-user affordances are form dependent [17] ie a different structure will give rise to different affordances and leads to the idea of positive affordances aiding a required potential use of an object and negative affordances which hinder the potential [17]. However, Gibson [10] identified a different type of affordance that was independent of perception, what Maier [17] calls

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‘artifact-artifact affordances’ regardless of use. Stamper [18][31] expanded the concept of affordances using organisational semiotics to develop a method of modelling rules/behavioural norms that relate to human motivation for actions and transformations in formal and informal processes. This provides a model relating to the behaviour of both physical objects and intangible entities such as social behaviours ie it directly relates to tangible and intangible resource behaviours in our capability model. Norman sees affordances as ‘referring to both potential and actual properties of a designed device [19] and uses the term ‘perceived affordance’ to describe the interaction of objects with people. This approach enables us to differentiate the perceived affordance and hence capability between one designed object and another eg design implications of man-made objects such as handles optimised for pulling via a gripping section or plates for pushing [32]. Affordances are generalised as the properties of entities in ontology [18]. Both types of affordances enable a description of the world based on motor and perception abilities of human agents [15]. You and Chen highlight that affordance is the potential of an object (ie resource) and quote Krippendorf’s affordance as the ‘operational meaning of objects’ [21]. Table 2. The Variety of Affordance Definitions

Definition

Ref

an invariant combination of properties of substance and surface taken with reference to an animal property of an object independent of perception' and a' subjective property depending on the perception of the viewer ' Affordances are qualities of entities of the environment: quality, sensory impression and potential action Person-object 'Affordances are form dependent'

[10]

affordances describe the world and objecct properties usinge the motor and perceptive abilities of the agent' affordance refers to both potential and actual properties of a designed device affordance is every thing physical or social we perceive'

[10] [12] [17] [15] [19] [18]

afforfdance - the perceivable potentiality of the human-scaled object [21] that supports the intended action without requiring memory, inference, or interpretation affordance is the operational meaning of objects [21]

We can say affordances represent potential properties of object-agent and objectobject interactions and hence relate to our capability definition as a function of the resource objects and of the actions (process) by which the resources interact.


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Affordances as Object-Resource Interactions

To model capability in detail we develop the theory of affordances to represent the capability-affordance model, focusing mainly on physical affordances in this paper. Turvey’s model [20] explains the affordance mechanism depends on the interaction of object resources at the point at which they interact. For example the surgeon must hold the syringe to use it, or must communicate to a nurse etc. Resource interaction or transformation requires an interface between resource objects and so we use the term ‘affordance pair’ to highlight this mutual dependency. The affordance between two resources Rp and Ra, where Rp is the passive resource Ra is an active agent resource can be represented as Afap. Afap is a tuple of a passive resource and a driving agent and represents the interface interaction or affordance of the pair of object resources. This interaction property is different from the properties of the separate object resource systems. The affordance Af is represented as an ordered pair Afap = f(Ra,Rp). We use the rule that the active or driving resource should precede the passive resource undergoing transformation (in cases where this is perceptible).

2

Objective and Subjective Affordances

Affordances are both an objective property of an object independent of perception, but also a subjective property depending on the perception of the viewer. This section unifies these two views to enable capabilities to be modeled by affordances. 2.1

Objective Affordances

Affordances existing independent of perception are inherent objective or ‘passive ‘structural properties of the environment or object resource. For example a solid floor objectively and passively affords support of objects placed on it. Conversely, the perceptive view of affordance, relies on an agent identifying how an object resource could be used to achieve a transformation. Objective affordance relates to a capability of interacting resources that is happening. Subjective affordance relates to the capability of what could happen between object resources. Passive objective affordances (OA) are what keep object resources in an unchanging, ie motionless state of equilibrium. These affordances are not perceptions or possibilities , the possibility has already happened and we are observing ‘executed affordances’. The interface relationship has been ‘actualised’ and the affordance is acting in a state of equilibrium. Objective affordances relate to the qualities of passive resources in stable equilibrium at an externally perceived level ie excluding semiotics at a microscopic level as Noth’s view [33]. Any tangible resource has a set of state determining qualities that are required to be balanced in order to execute a passive objective affordance. This comprises, for example, the force experienced by an object, its spatial position and other quality attributes. Objective affordance therefore relates to actual physical qualities or properties of the natural behaviour of the object resource such. For example the laws of natural science such as physics (Newton’s laws) chemical and biological laws etc. The passive objective affordance is a stable state where:

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• • •

Transformation forces on the object are balanced The object’s special position is unchanging The object’s attributes are not changing (chemical composition, dimensions etc).

The objective affordance transformation mechanism is in a state of stable passive equilibrium. An example is the weight of a cup due to gravity is balanced by its structural strength and reactive force (Newton’s third law) of the surface on which the cup sits. The equilibrium is stable as the potential transformation mechanism eg the forces are in balance and the cup resource object is a passive resource and therefore possesses no means of motive force or transformation mechanism capability of its own. This suggests that affordance depends on the mechanism of transformation at the interface of the affordance pair. In this example the affordance interface mechanism is the force of gravity modelled by Newtonian mechanics, ie force is equal to mass x acceleration due to gravity, to stably maintain the cup in contact with the table. Objective passive affordance relates to what Norman [19] calls the intrinsic properties of things ‘innate’ affordances. This passive objective affordance exists without the need for perception. This relates to Turvey’s [20] view of affordance as an environments capability to support an activity. Affordance = the disposition (of the object) ie force balance on table). This equilibrium state of object resources continues unless acted upon by an external force (Newton’s 1st law). This brings us to subjective affordances, ie what potential resource interactions and transformations are possible. 2.2

Subjective Affordances

Gibson saw affordances as ‘perceptibles’ [10]. These are subjective-affordances depending on perceived actions and the possible ways in which resource objects could interact or be made to interact by driving resources. This requires an agent to intelligently perceive the affordance or possibility for using a resource object to make a substantive transformation and has an intention and plan to use it. We use the term ‘subjective’ as the identified affordance depends on the driving resource’s goals and motivations in the intended action and also critically on the selection of the ‘path of action’, sequences or process. These perceived ‘action affordances’ relate to planned actions in business processes or human visions for action. Subjective affordance depends on agent’s perceptions of how properties of the object may be used to achieve a goal. The intention or goal of the outcome must complement the affordance [12]. The goal; also helps i) to define a set of best action paths or processes that could be used and ii) to define the value or client benefit of the result expected to be achieved on completion of the action. However, affordance depends on the interface between an affording object and the driving agent that is attempting to use it. Affordance is the property of interaction of two entities or systems that enable an action to achieve a goal. It depends on the transformation mechanism that changes an object resource state from state a to a’ eg physical, chemical or other force. 2.3

Relating Objective and Subjective Affordances

Any resource is a collection of capability opportunities or affordances which depend on how the resource is related to another resource and what affordance mechanism is


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to be employed to achieve the state transformation and meet the goal. A simple passive resource such as a cup can be used as a doorstop (weight affordance mechanism Af1), as a missile (kinematic mechanism of a thrown cup affordance Af2), as means to drink (the cup affords drinking Af3). But each affordance requires the cup (i) to be used in an affordance pair interfaced with something else (j) by a driving resource ie a resource R is a function of an affordance pair f{Afij}. Turvey [20] in his nest building argument provides support for the driving and passive resource view [5] by differentiating between the properties of an animal (or agent) affordance which seeks to identify the affordance properties of the environment ie how the resources can be used by the person, (ie via manipulation, construction, imagination etc). Affordance is the capability of the transformation mechanism necessary to achieve the goal. The agent driving resource in a process seeks to identify the disposition of the resources (objective affordance) to achieve the transformation action required by the process (subjective affordance). In summary, objective affordances refer to executed and operational affordances ie an on-going system interface relationship where the mechanism of transformation becomes an equilibrium mechanism. The path followed by the resource is completed and meets the goal eg affording structural support. In contrast the subjective affordance has to be perceived or planned and therefore the mechanism and path may change and so may the outcome or actualisation and ability to achieve whatever goal is perceived. Subjective and objective affordances don’t just depend on the transformation mechanism, but also on the way or path taken to actualise or execute the affordance. The ‘cup affords drinking’ [18] only if a) the cup is grasped firmly without obscuring the opening, b) if the cup is brought to an animal’s mouth by an action that changes the position of the cup to the proximity of the mouth c) it is tilted to enable gravity to act on the fluid and or the animal sucks to aid the fluid transfer. Subjective Affordances

Objective Affordances

R1

+

Active resource

Reaction force R2

Percieved Possible interactions?

R1

Mechanism2, path2

Weight

Resource R2

Goal G: cup is supported on table Value: cup is immobile

Mechanism

Substantive Affordance

Path

Mechanism1, path1

Objective affordances Subjective affordances

Mechanism3, path3

Affordance is actualised Transformation mechanism is active Action path is actualised

• • •

• • •

Affordance is perceived Transformation mechanism is potential Action path is potential

Fig. 2. Objective vs. Subjective Affordances

To achieve Goal G, Value V

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3

Analysing Affordances

3.1

Affordance Modelling Notation

Capability rarely comprises one affordance, but depends on multiple affordances. In the medical example of a capability ‘to anaesthetise a patient’ the goal is: ensure the patient has a controlled unconscious state. We will consider a sub-capability, a single action of administering a medical injection and explore the interaction between the anaesthetist, the syringe and the patient. Using the high level capability model [5] the anaesthetist is the active agent driving a passive resource ( the syringe) to execute the capability Ci of injecting an anaesthetic drug into a patient via a process/action Pi that results in the value of ‘patient anaesthetised and can be safely operated on’. We denote the affordance by the term Axy where A is the affordance and x and y refer to the two elements in the affordance pair. We use an underline to refer to the fact that an affordance relates to a vector dynamic transforming action or a static force balance and hence has magnitude and direction. A line terminated by balls, with each ball resting of the relevant element or component shows the relationship between the two components of the affordance system. The syringe is an example of a designed device [22]. It comprises 3 components (excluding cap) a plunger, body and needle together forming a passive system of affordances (see below). Anaesthetist’s hand

Ahp

syringe plunger p

Apb syringe Body b

Anaesthetic fluid

Afb Abn

Anf

syringe needle n

Ans Afp

Fig. 3. Syringe Affordance Pairs

3.2

Affordance Chains and Objective Affordances

Multiple affordances can be associated with a particular subsystem [17]. The affordance of the syringe is a combination of the affordance of the component subsystems comprising the syringe ie Af (passive) is a function f{Afs}. The passive affordances of the syringe relate to the mechanical structure and engineering mechanics of the syringe. There are a number of passive objective affordances. The Plunger-Body Affordance, Apb, ensures the plunger is retained in the barrel and slides easily up and down it by the affordance pair. The affordance mechanism AM depends on force transmission and friction and the affordance path is the axial plunger motion. In the Hand-Plunger affordance Ahp, the plunger has a tube (with a seal) and


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top to it which supports the affordance of ‘holding’ the top of plunger in order to exert downward force (Ahp). This downward force acts on any fluid contained by the syringe body (Apb). This affordance (affords force transmission to fluid) is dependent on the properties of interaction of the plunger and body. The plunger must slide and be a secure fit vs. body, must be made out of materials not affected by the fluid to be injected etc. The act of compressing an incompressible fluid forces the fluid out through needle via an affordance Anf, ie the needle –fluid affordance. An affordance property of the needle (affords fluid transportation) is that its hollow tubular nature directs the anaesthetic and is chemically inert vs the fluid (a potential negative affordance). A further affordance exists between the needle tip and the patient’s skin Ans. The small surface area at the point enabling a small force on the plunger to be safely and effectively transmitted (due to mechanism of mechanics ie pressure is force/area) as a large pressure on the patient skin surface affords penetration of the skin. However this of course is not possible without the subjective affordances of the active driving resource, the anaesthetist who is responsible for identifying the target vein and ensuring the tip of the needle enters a vein etc in specific path. We define an affordance chain as a connected and related set of affordances (and hence affordance mechanisms) acting together as a system at a point in time to achieve a specific goal of enabling a new macro affordance mechanism, ie the capability. Ca = affordance chain = f (A1,2, A2,3, A3,4 …Ai,j) .

(2)

Where j= i+1 in a sequence from driver to final resource. The idea of affordance chain allows us to link the micro level affordances to the actions that we define as part of the process referred to in our definition of capability. 3.3

Subjective Systems of Affordances

There are three affordance pairs that depend on the interaction of a passive and a driving resource. Aph refers to affordance between the plunger and the anaesthetist’s hand. The needle in contact with the patient’s skin provides another affordance pair Ans. One half of the affordance pair comprising the anaesthetist and patient can both act dynamically, deontically and independently, unlike the passive syringe which requires interaction with both these active resources, one of which – the anaesthetist, is driving the overall capability transformation. Another affordance may be the anaesthetists second hand resting on the patient to steady the needle – this is a subjective agent affordance, but is not illustrated for clarity. The third affordance pair Afp is the interaction between the anaesthetic fluid f and the patient’s body physiology p. Here the driving resource is the anaesthetic which has a sleep inducing transformation mechanism, if of the right fluid composition and if the delivery or action path – ie into a vein is efficacious.

4

Linking Capabilities and Affordances

Based on the above discussion we can rewrite our original capability model. Capability = f(resource interaction, process of interaction). Any capability depends

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on a transformation based on the affordance mechanism and an affordance path, both of which are functions of the set of resources {Ri} needed to deliver the capability: Capability = f(Affordance mechanism AM(Ri), Affordance path AP(Ri)). 4.1

(3)

The Affordance Mechanism AM

The affordance mechanism describes the potential transformation at the interface between the two object resource systems Rp and Rq and its properties that enable the transformation. Affordances are ‘actualised’ or realised by an affordance mechanism AM, that represents the transformation. Our affordance mechanism transfer function relates to what Turvey call’s ‘ function j’ in his example of how light is bent by the refractive properties of crystal refraction [20]. The affordance mechanism can be represented as a mathematical function eg in this case a refraction equation or a description. Another example is Kornhauser’s affordance example [20] of a bird flying onto a branch. This depends on the affordance mechanism of the law of conservation of momentum that transfers the bird’s momentum to the branch, ideally with not enough momentum to break the branch, but enough to stop the bird safely. The Critical Affordance Factor. In Turvey’s examples [20], the successful bending of a light ray (refraction) is subject to what we can call the critical affordance factor ‘fc’ being within the correct window to enable the affordance to actualise or manifest itself. Similarly Warren’s work on affordance potential to climb a stair similarly identified the riser height, [30] as a limiting, or what we call, critical affordance factor. Affordance factors relate to the critical metrics within which the affordance will work. Successful refraction is dependent on the ‘affordance path’ ie if light ray strikes crystal to obliquely it will be reflected – dependent on the critical affordance factor – the angle of the light with respect to the normal to the crystal surface. Similarly for Kornhauser’s bird affordance example [20],the bird’s velocity (assuming a straight unimpeded flight) has a velocity safety envelope that determines safe/unsafe momentum transfer. The bird’s speed is a critical affordance factor in the affordance mechanism that will determine the safe actualisation of the affordance and the path of action of the bird another. 4.2

The Affordance Path AP

The affordance path should describe the space-time interaction of how the resource systems must be brought together to enable the interface transformation mechanism specified by the affordance to occur. It is the path followed by actions to enable affordances. An affordance path AP is the set of possible actions that could be taken to enable the affordance mechanisms to act and execute the capability. For a capability the affordance path is a process or sequence of actions necessary to realise the capability. It refers not just to a set of closed actions within one activity (like the affordance chain), but a process of linked, ordered, but discrete actions as part of a business activity. This will comprise physical or substantive actions eg the act of injecting an anaesthetic, as well as non substantive ie semiological actions [22] eg verbal communication or inspection


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actions by the anaesthetist as a control on the action of injecting and assurance of the resulting value/goal achievement. We can say: AP is set of actions within an ordered sequence, it contains actions represented affordance chains, which can be substantive (physical) and semiological (eg thoughts). Affordance path = ∑ subst. affordance chains +semiological affordance .

(4)

Affordance paths P relate to Luck and d’Inverno’s [11] total plan as a sequence of actions and Brown's [22] ‘operations’ and to our capability ‘process’. The anaesthetist has an almost infinite range of possible affordance paths in wielding the syringe. We can simply prod the patient with the syringe etc. The quality of the action path chosen will impact the efficacy of the affordance mechanism and determine how well the capability transformation is achieved. 4.3

Mode of Action M

The syringe tool has a designed way of being used – ie a set of preferred potential affordance paths. The syringe is a passive, but designed resource or a device (D) and must be guided by the surgeon in a specific designed way, at a certain angle and depth to inject into the vein to meet the desired goal of anaesthesia. This optimum path (in space and time) is the mode M(Ds,E) [32] of deployment of the syringe device. The mode specifies the ideal and optimised metrics of the actions involving a designed object resource, eg the procedures for using the syringe. The mode may come from tacit best practice knowledge or following procedures or documents defining usage procedures. For example procedures for the identification of the vein and control inspection of the injection process, although these may be in the form of tacit knowledge and experience. The mode refers to an optimum affordance path for the affordance chain ACs of the syringe: Mode M (Ds, E) is the optimum f(affordance paths P1-Pn) of actions mentioned earlier. But the optimum mode is dependent on the anaesthetist’s sensor affordances (sight/touch etc) and his tacit knowledge/conceptual model of the environment. The mode defines certain action behaviours to ensure the affordance mechanism is optimised eg: ensure syringe is in the correct location to meet a vein, it doesn’t slip (negative affordance) and it reaches the correct depth to interact with the vein. Deviations from the mode, the anaesthetist using the syringe in a different way and perhaps using different resource affordances may not deliver the desired transformation goal of anaesthetised patient and the value they require to perform the medical intervention.

5

The Capability Affordance Model

We can now integrate the affordance model with the high level capability model. To model the capability we require i) a model of the overall capability ii) a model of the process/path ii) a model of the mechanisms iii) a means to quantitatively compare capabilities.

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A Model of Capability

The process associated with capability (fig 1) is an affordance path which involves the coordination of individual activities that involve actions to transform resources. The capability at the lowest resource interaction level requires an affordance mechanism and an affordance path that work within a range of critical affordance factors. Some resources may work closely together like components of a device or agents performing in the form of a connected chain of affordances. As capability is a function of resource interactions and processes, at the lowest level it is also a function of the affordance mechanisms AM (resource interaction model) and the affordance path or between the resources AP. Each of the affordance mechanisms relates to an interaction between two or more resources and is dependent on the critical affordance factor(s) for the affordance to be realised. The capability of a system of business resources is therefore dependent on the sum of the affordances in the transformation process p working together. The capability of a resource system (high level capability) depends ultimately on the resource interaction capability between two or more resources. The lowest level of capability is a property of the affordance between the resources (affordance system) that depends on the mechanism and space time path of interaction between the resources. For a specific resource capability there are metrics associated with properties p of the interacting resources (critical affordance factors) that need to be in a defined range for the affordance and hence the capability to meet the goal and enable the transformation that delivers the goal of the capability (figure 4). Capability System Process P

Activity a

Resource Ri

Affordance Chain

Affordance Path

Activity b

Resource Ri+1

Resource Interaction Capability Activity b Affordance

Affordance Mechanism AM

Critical affordance factors

Activity b

Affordance System Affordance Path AP

Critical affordance factors

Fig. 4. The Capability- Affordance Model (Mechanism and Path)


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Using the models we can demonstrate how to analyse the capability. Firstly we identify the high level capability model, the system of resources and process to implement the capability as in figure 1. Using our example, the Capability Ci of anaesthetising a patient via a process/action Pi results in the value v of ‘induced immobile sleep state in patient’. The capability depends on the resources used and their capability and the way (path ) in which they used. When using a syringe there are four interacting resource systems. We can say; Capability of anaesthetising a patient is a function of{resource system}ie f(anaesthetist, syringe, anaesthetic, patient). Some resources (eg a syringe) a are set of interacting passive objective affordances, ie a passive resource that must be driven by active resources –the anaesthetist. The capability process will include the ‘syringe injects anaesthetic’ action (the composite affordance chain), but also relates to further subjective affordances including semiological affordances in inspecting the patient and controlling the syringe action and the way the patient reacts. The capability depends on the sum of the affordance mechanisms and paths working together in each part of the process. Each affordance represents a capability of a resource. The capability of a specific activity, eg injection, is dependent on a tuple, or affordance chain by which the anaesthetist acts on the syringe which in turn acts on the fluid which in turn acts on the patient through a number of transformation mechanisms. 5.2

Capability Processes or Affordance Paths

To model the capability process or affordance paths supporting the capability we use ellipses for active resources with driving agent resources as a solid colour. Passive resources are represented as boxes with designed passive resources as a shaded box. Action affordances use the solid dumb-bell notation and intangible semiosis driven affordances involving perception eg anaesthetist identification of vein and control actions via a dotted line as below.

Process (affordance paths)

Key Driving Resource

1/2. Substantive Action: dilute/load drug

Anaesthetist

3. Semiosis Action: identify Vein and target site

Anaesthetist

injection device

drug

diluent

Goal G : drug of correct strength prepared

4. Substantive Action: inject anaesthetic

Anaesthetist

5. Semiosis Action: Control inspection

Anaesthetist

6. Substantive Action: Swab site

Anaesthetist

injection device patient

Passive resource

Passive resource system patient

Goal G : vein Identified

Active resource

Goal G : anaesthetic injected

patient

Goal G : check anaesthetic Injected correctly

patient

Goal G : patient wound cleaned

Solid resource (no subsystems) Eg a rock

Componentised resource (with subsystems) Eg syringe

Substantive Affordance system Ie substantive force/action transformation Semiological Affordance system

Mode – procedures specifying the optimised metrics and states of the actions

Fig. 5. Capability Path Diagram: to Anaesthetise a Patient (syringe system)

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5.3

Capability Affordance Mechanism and Critical Factors

We now need to identify the affordance mechanisms and any critical affordance factors that affect the capability being achieved. The capability affordance system model can be used to identify the affordances and mechanisms based on analysis of the earlier path model, as in the table below. In the table we have expanded the syringe affordances to illustrate the range of affordances and factors that support the capability. The anaesthetist’s operation of the syringe (subjective affordance) is dependent on the critical affordance factors for the correct angle and path of the needle. Also note how important the correct dilution of the drug might be. This approach can be used to provide a detailed analysis of the critical factors especially where subjective affordances are involved. Table 3. Capability-Affordance System Model; 'to anaesthetise patient' using a syringe Ref

1

Activity

Affordance type

dilute drug

2

dynamic, subjective dynamic, subjective

load drug 3

4a

4b

5 6

Afforda Affordance Description nce Ref.

Add

Ald dynamic, subjective

identify vein and target site

Aiv dynamic, subjective

Anaesthetist operates syringe dynamic

Aos Ahp

dynamic

Apb

dynamic

Afb

static

Afn

static

Ans

dynamic

Afp

Affordance Mechanism (AM)

knowledge of drug afford safe dilution

solvent effect of diluent affords dilution vacuum caused by withdrawal of affordance to move the anaesthetic plunger affords drawing up drug fluid into the syringe into syringe knowledge of vein and physiology and experience combined with perception afford correct anaesthetist perception, knowledge identification and experience

Affordance Path (AP)

spatial path of diluting the drug spatial path of drawing up drug and its contact with the fluid

Aci

frictional sliding force of plunger seal on syringe body fluid force containment and expansion towards needle tube fluid force expansion towards needle tip/skin affords skin penetration pressure = force/area at the tip, where force Fns achieves steady penetration Affords anaesthesia (anaesthetic induced sleep via chemical body interface) interaction between anaesthetic and nerve/brain centres anaesthetists skill affords correct perception of correct vs. incorrect check on injection quality injection

Ass

remove blood leakage

dynamic

Affords sliding and force transmission Affords containment and force transmission Affords fluid path transmission

dynamic swab site

5.4

physical removal of blood

slider slides 1cm fluid moves down body fluid moves down needle tube needle penetrates skin to a depth d

Critical affordance factors (fa)

diluted drug

correct diluent used and volume of diluent

syringe body full of drug

plunger speed, avoiding air bubbles

vein injection site eye tracking of vein identified align syringe ion injection anaesthetists skill to afford correct site move plunger to end angle depth and pressure on anaesthetist perception, knowledge of travel (eg1 cm) syringe needle positioned syringe plunger and experience in vein plunger affords pushing and apply force Fhp on plunger acting move plunger to end of plunger location relative grasping along axis of plunger travel (eg1 cm) to body

Syringe Injects anaesthetic

control inspection

Variables changed

visibility of vein, support of body

depth of needle, angle of needle and force applied Fhp, f1 > Qss, with a shorter affordance path, fewer negative affordances and less critical affordance factors. Actual relative efficiency and effectiveness could be calculated using the capability-affordance model, subject to available data. This would enable quantitative and qualitative objective decisions about which subjective capability to use and what critical factors to manage and control to ensure the capability is achieved.

7

Summary and Conclusions

This paper extends and links previous work on capability and addresses the three identified gaps in the high level model. We have shown a) how capability behaviour and qualities can be modeled and analysed at a consistent and detailed level by integrating the high level model of capability with affordance models at the resource interaction level. By developing the concept of objective and subjective affordances capability can be modeled as a tuple of a set of resource affordance mechanisms and action paths, dependent on one or more critical affordance factors. We identified an affordance chain of subjective affordances by which affordances work together to enable an action and an affordance path that links action affordances to create a capability. To resolve gap b) we introduced model that defines the mechanism and path that underlies capability. We showed how Affordance Modeling Notation, AMN, provides a visual method of representing the affordances in a system of interacting resources. Gap c) a consistent quantitative and qualitative comparison of capabilities is addressed by the proposed model of efficiency, effectiveness and quality that enables the performance of a capability and its constituent affordances to be measured and modeled. The capability of syringe and Powderject based anaesthetic systems was compared and evaluated as an example of the approach. 7.1

Further Work

The method can be time consuming, so simplifications need to be investigated. Further work is now in progress to test the practicality of the method with larger capability sets and a greater study of semiological affordances and their relationships.


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The comparative measurement of efficiency, effectiveness and quality of capability systems in this paper is still rather cursory and requires more practical examples where metrics are available. Work underway in this area includes the analysis of capabilities of nurses to manage patient pain, comparing the current process of nurses asking patients about their pain at specific times of the day vs. an automated patient driven pain sensor approach. This requires larger scale relative assessment of nurse and patient capabilities and the technology enablers used in each case. Acknowledgement. The author would like to acknowledge the support of the Royal Berkshire Foundation Trust Simulation Centre for his Honorary Senior Lectureship in Health Informatics. Also to acknowledge his colleague Dr Debbie Rosenorn-Lanng Director of the Simulation Centre for the opportunity to witness and analyse medical procedures.

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