12 Apr 2013 ... System Dynamics modelling process. 12th March 2013. STERMAN, J. D. (2000)
Business dynamics : systems thinking and modeling for a ...
Systems Modelling and Qualitative Data Dr Mike Yearworth Reader in Engineering Systems
12th April 2013
! Purpose • Presenting an approach to grounding systems modelling, by… • Describing system dynamics modelling • Describing what I mean by qualitative data analysis – specifically grounded theory • Bringing the two together • Practical details – using CAQDAS and some properties of binary matrices
• Presenting results • Debating • Methodology – justifying the approach using arguments of multimethodology
12th April 2013
2
! Origins of Dynamic Complexity • Feedback! A causes B, B causes C, … causes A • Negative feedback (balancing) – goal seeking, control system • Positive feedback (reinforcing) – unlimited growth until bounded by exogenous factor(s) • Combined: patterns of +ve or –ve loop dominance, • Exponential, goal seeking, damped oscillation, limit cycles, chaotic behaviour
• Delays complicate behaviours e.g. in –ve feedback loops • This is not detail complexity, system structure can be quite simple yet still produce complex dynamic behaviour
12th April 2013
3
! What is System Dynamics?
basic Forrester construct
12th March 2013
! System dynamics modelling elements • Causal Loop Diagrams (CLDs) • Used to surface mental models about the behaviour of elements (variables) of the system expressed as causal relationships and feedback loops
• Stocks and Flows (S&F) Maps • Describe the structure of the system in terms of flows and accumulations of things
• System Dynamics (SD) Models • Combined CLD+S&F which describe the dynamic behaviour of a system
• Model boundary chart • Endogenous/exogenous/excluded variables
• Sub-system diagrams • Overall architecture of a model, comprising sub-systems and flows of things between sub-systems 12th March 2013
! System Dynamics modelling process Real World
Decisions
Information Feedback
1. Problem Articulation (Boundary Selection) 5. Policy Formulation & Evaluation
4. Testing
Strategy, Structure, Decision Rules
xxx X
2. Dynamic Hypothesis
3. Formulation
Mental Models of the Real World
STERMAN, J. D. (2000) Business dynamics : systems thinking and modeling for a complex world
12th March 2013
! Causal Loop Diagrams (CLDs) • Relationship between variables represented by arrows linking them • causal relationship • can be positive or negative + + Investment
Work Completion Rate -
Jobs
Project Completion Time
Number of Project Staff Cost of Fuel
+
Car Journeys
+
Management Complexity
12th March 2013
! Causal Loop Diagrams – textual analysis • “Increased government investment in SWRDA will lead to the number of jobs created in Bristol going up” • “Raising the tax on petrol will reduce the overall number of car journeys” • Management meeting… • Director – “Increasing the number of project staff will improve our work completion rate and improve our project completion times.” • Project Manager – “But increasing the number of staff in my team will make my job way more complex and probably lead to worse completion times.”
12th March 2013
! Causal links and feedback loops + W
Z -
A
B
+ X
Y
Read as W causes Z with +ve link polarity or mathematically as (∂Z/∂W>0). If the cause increases, the effect increases above what it would otherwise have been. Read as A causes B with ,ve link polarity or mathematically as (∂B/∂A
