Workforce modellingAn introduction

Sally BrailsfordSchool of Management

University of Southampton, UK

TSRC Workforce day, London 22 September 2011

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

What is a model?

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!

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

Influence diagrams

• As A increases, B also increases

• As A increases, B decreases

A

B

A

B

+

–

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

A university example

Students enrolled

Research paperspublished

Reputation ofuniversity

Studentapplications

Staff stress levels

-

+

+

+

+

A balancing feedback loop

Students enrolled

Research paperspublished

Reputation ofuniversity

Studentapplications

Staff stress levels

-

+

+

+

+

Behaviour over time

time

Number of students

A football example

Money to buy topplayers

Man Utd leagueposition

Satisfaction ofArsenal fans-

Goals scored permatch

+

+

Gate and TVreceipts

+

+

A vicious circle

Money to buy topplayers

Man Utd leagueposition

Satisfaction ofArsenal fans-

Goals scored permatch

+

+

Gate and TVreceipts

+

+

Behaviour over time

time

Success of Man United

Negative Feedback Loops

• Target seeking (control) loops – try to correct a gap between target and actual

• Stabilising• Examples - body temperature and sweating

Positive Feedback Loops

• Self-reinforcing, unstable, spiral out of control• Examples

• Population / Birth rate• Bank balance / Rate of investment

A health example: waiting lists

Occupancy ofhospital beds

Referral rates Waiting lists

+

+

-

Hospital waiting lists

Occupancy ofhospital beds

Referral rates Waiting lists

+

+

-

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

We need more beds!

Occupancy ofhospital beds

Referral rates Waiting lists

+

+

-

Money for extrabeds Political pressure

+

+

+

-

Unintended consequences

Occupancy ofhospital beds

Referral rates Waiting lists

+

+

-

Money for extrabeds Political pressure

+

+

+

-

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

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

Inflow and outflow

Water inbathtub

Water tankDrainagesystem

inflow throughtaps

Outflow throughplughole

Stock

Tap Flow

Add more detail …Water inbathtub

Water tank Drainagesystem

inflow throughtaps

Outflow throughplughole

Watertreatment

center

Sewagetreatment center

flow to house

Natural wastage

Reservoir

Rainfall

flow aftertreatment

outflow

Flow into sewage

External source or sink

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!

SD for workforce planning

Add more detail

Slide 28

Slide 29

A real-life example

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

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

To get you started

Slide 32

Third sectorworkforceSUPPLY

Ageing population

Governmentfunding cuts

Public sectorpension cuts

Third sectorworkforceDEMAND

Private sector jobavailability

+

+

Third sectorworkforceSKILLS

+

Slide 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