1. Workforce modelling
An introduction
Sally Brailsford
School of Management
University of Southampton, UK
TSRC Workforce day, London 22 September 2011

2. Aims of today’s session
• What is a model?
• Brief introduction to System Dynamics modelling
• Using System Dynamics to model workforce planning
issues
• A practical exercise
Slide 2

3. What is a model?

5. Some myths about modelling
• You need to be a maths wizard to do it
• You need to be a computer geek to do it
• You need expensive software
• You need lots of data
• It is difficult!

6. System Dynamics
• Modelling technique which can be done on paper or
using a computer
• Does not always need much data
• System structure determines behaviour: i.e. the way
that the separate components of any system relate to
and affect each other determines the behaviour of the
system as a whole
• System behaviour may be counterintuitive
• Feedback is an important feature

7. Influence diagrams
• As A increases, B also increases
A + B
• As A increases, B decreases
–
A B

8. Feedback loops
• Negative or balancing loops have an odd number of
“–” signs
• Positive or reinforcing loops or vicious circles have an
even number of “–” signs
• Loops or causal chains show how changes (even quite
small) in one part of the system can eventually lead to
unexpected changes (possibly quite large) in other
parts of the system

9. A university example
+
Staff stress levels
Students enrolled
+
-
Research papers
Student published
applications
+
Reputation of
university +

10. A balancing feedback loop
+
Staff stress levels
Students enrolled
+
-
Research papers
Student published
applications
+
Reputation of
university +

11. Behaviour over time
Number of students
time

12. A football example
+
Goals scored per
match Money to buy top
players
+
+
Man Utd league
Gate and TV
position
receipts
+
Satisfaction of
- Arsenal fans

13. A vicious circle
+
Goals scored per
match Money to buy top
players
+
+
Man Utd league
Gate and TV
position
receipts
+
Satisfaction of
- Arsenal fans

14. Behaviour over time
Success of Man United
time

15. Negative Feedback Loops
• Target seeking (control) loops – try to correct a gap
between target and actual
• Stabilising
• Examples - body temperature and sweating

16. Positive Feedback Loops
• Self-reinforcing, unstable, spiral out of control
• Examples
• Population / Birth rate
• Bank balance / Rate of investment

17. A health example: waiting lists
Occupancy of
hospital beds
+
+
Referral rates Waiting lists
-

18. Hospital waiting lists
Occupancy of
hospital beds
+
+
Referral rates Waiting lists
-

19. Lessons from this simple model
• This is a stable system!
• As waiting lists rise, GPs seek alternatives to hospital
admission
• Suggests that waiting lists play a useful regulating
function by controlling admission rates and keeping
bed occupancy down

20. We need more beds!
+
Money for extra
beds Political pressure
+
-
Occupancy of
+ hospital beds
+ +
Referral rates Waiting lists
-

21. Unintended consequences
+
Money for extra
beds Political pressure
+
-
Occupancy of
+ hospital beds
+ +
Referral rates Waiting lists
-

22. Supply-induced demand
• When supply of beds is scarce and waiting lists are long,
demand falls as GPs do not admit patients to hospital but try
other forms of treatment – the effect of the balancing loop
• When supply of beds increases, referral rates increase again
• Leads to a vicious circle – mitigated of course by the actual
increase in beds
• Some elements (e.g. bed occupancy) appear in several loops
• To determine the actual net effect, we need some numerical data
to quantify the model

23. Quantitative SD: stock-flow models
• Implemented in computer software
• A stock is like a bathtub: water flows into the bath
through the taps and flows out through the plughole
• The rate of flow is governed by taps or valves
• Material in stocks is continuous, like water - even if
we are dealing with individual items like people

24. Inflow and outflow
Stock Tap Flow
Water in Drainage
Water tank
bathtub system
Outflow through
inflow through
plughole
taps

25. Add more detail …
Water in Drainage
Water tank
bathtub system
Outflow through Natural wastage
inflow through
plughole
taps
flow to house Flow into sewage
Water Reservoir Sewage
treatment flow after treatment center
center outflow
treatment
Rainfall
External source or sink

26. System Dynamics: summary
• Powerful methodology for problem structuring:
constructing diagrams is an iterative process, carried
out with all the stakeholders
• Eliciting opinions from stakeholders and constructing
the diagram is a useful exercise in its own right
• Can gain helpful qualitative insights into system
behaviour even without any data
• Software can be used to automate detection of all the
feedback loops in more complicated systems
• It’s not always necessary to build a quantitative
model!

27. SD for workforce planning

28. Add more detail
Slide 28

29. A real-life example
Slide 29

30. Use of the model
• Used in 2010 for policy-
making by the Sri Lankan
Government
• The Ministry of Higher
Education used the model to
determine how many
university dental school places
to fund over the next ten years
• The Ministry of Health created
400 additional Government-
funded posts over the three
years 2012-14 based on the
model findings
Slide 30

31. Exercise
• Working in small groups, identify some key factors
which you think will influence the supply of, the
demand for, and the skills base of, workers in the
third sector over the next five years
• Then construct an influence diagram showing how
these are connected
• See if you can identify any feedback loops!
Slide 31

32. To get you started
Third sector
+ workforce Government
DEMAND funding cuts
Ageing population +
+ Third sector
workforce
SUPPLY
Public sector
Private sector job pension cuts
availability
Third sector
workforce
SKILLS
Slide 32

33. Recommended reading
Business Dynamics: Systems Thinking and Modeling for a
Complex World with CD-ROM by John D. Sterman
McGraw Hill, 2000
Hardcover £45.59 from Amazon
Strategic Modelling and Business Dynamics: A Feedback
Systems Approach by John Morecroft
John Wiley & Sons, 2007
Paperback £37.04 from Amazon
Thinking in Systems: A Primer by Donella H. Meadows
Chelsea Green Publishing, Vermont, 2008
Paperback £10.80 from Amazon
Slide 33