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Model Validation as an Integrated Social Process. George P. Richardson Rockefeller College of Public Affairs and Policy University at Albany - State University of New York [email protected] What do we mean by ‘validation’?.

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Model validation as an integrated social process

Model Validation asan Integrated Social Process

George P. Richardson

Rockefeller College of Public Affairs and Policy

University at Albany - State University of New York

[email protected]


What do we mean by validation
What do we mean by ‘validation’?

  • No model has ever been or ever will be thoroughly validated. …‘Useful,’ ‘illuminating,’ or ‘inspiring confidence’ are more apt descriptors applying to models than ‘valid’(Greenberger et al. 1976).

  • Validation is a process of establishing confidence in the soundness and usefulness of a model. (Forrester 1973, Forrester and Senge 1980).


The classic questions
The classic questions

  • Not ‘Is the model valid,’ but

  • Is the model suitable for its purposes and the problem it addresses?

  • Is the model consistent with the slice of reality it tries to capture? (Richardson & Pugh 1981)


The system dynamics modeling process
The system dynamics modeling process

Adapted from Saeed 1992





The classic tests
The classic tests

Forrester 1973, Forrester & Senge 1980, Richardson and Pugh 1981


Validation is present at every step
Validation is present at every step

  • Conceptualizing:

    • Do we have the right people?

    • The right dynamic problem definition?

    • The right level of aggregation?

  • Mapping: Developing promising dynamic hypotheses

  • Formulating: Clarity, logic, and extremes

  • Simulating: Right behavior for right reasons

  • Deciding: Implementable conclusions

  • Implementing: Requires conviction!



Problem frame stakeholder map
Problem frame stakeholder map

High

Opposition

Low

Problem Frame

Low

Support

High

Weak

Strong

Stakeholder Power

Bryson, Strategic Planning for Public and Nonprofit Organizations


Power versus interest grid
Power versus Interest grid

High

Interest

Low

Weak

Strong

Power

Eden & Ackerman 1998


Pursuing validity in mapping
Pursuing validity in mapping

  • Think causally, not correlationally

  • Think stocks and flows, even if you don’t draw them

  • Use units to make the causal logic plausible, even if you don’t write them down

  • Be able to tell a story for every link and loop

  • Move progressively from less precise to more precise -- from informal map to formal map



These arrows mean and then
These arrows mean ‘and then’

  • We start with some understandings of the problem and its systemic context, and then we conceptualize (map) the system.

  • Then we build the beginnings of a model, which we then test to understand it.

  • Then we reformulate, or reconceptualize, or revise our understandings, or do some of all three, and then continue…


Arrows here are flows of material
Arrows here are flows of material

The words here represent stocks.

This is not a causal diagram.


Only this one is a causal loop
Only this one is a causal loop

No explicit stocks or flows, no clear units, but it tells a compelling story – It’s a good start.



Identical structure without explicit stocks and flows
Identical structure without explicit stocks and flows


Pursuing validity writing equations
Pursuing validity writing equations

  • Recognizable parameters

  • Robust equation forms

  • Phase relations

  • Richardson’s Rule: Every complicated, ugly, excessively mathematical equation and every equation flaw saps confidence in the model.



Complexity flaws destroy confidence
Complexity & flaws destroy confidence

  • P of int'l conflict =

    DELAY FIXED ((Lateral pressure/10*Military force effect/Trade and bargaining leverage + International conflict)/Lateral conflict break point, 1 , 0)

  • Flaws

    Complexity, discreteness, units confusion and disagreement, disembodied parameter, confusion of the effect of a concept [leverage] with the concept itself, and the wonder what keeps this probability between 0 and 1?









Phase relations
Phase relations

Constant Perceived Value suggests continually rising Resources, but that doesn’t seem correct


Phase relations1
Phase relations

Here, the Perceived Value of Integrated Information sets a planned level of resources


Pursuing validity fitting to data
Pursuing validity fitting to data

  • Generally, a weak test of model validity

  • Whole-model procedures

    • Optimization

  • Partial-model procedures

  • Reporting results

    • Graphically

    • Numerically: Theil statistics


Example of weakness of fitting to data
Example of weakness of fitting to data

  • Logistic curve

    • dx/dt = ax - bx2

  • Gompertz curve

    • dx/dt = ax - bx ln(x)





Presenting model fit numerically
Presenting model fit numerically

  • Theil statistics, for example

    • Based on a breakdown of the mean squared error:

    • 1 = Bias + Variation + Covariation



Learning from surprise model behavior
Learning from surprise model behavior

  • Have clear a priori expectations

  • Follow up all unanticipated behavior to appropriate resolution

  • Confirm all behavioral hypotheses through appropriate model tests (Mass 1991/1981)


Tests to reveal and resolve surprise behavior
Tests to reveal and resolve surprise behavior

  • Testing the symmetry of policy response (up and down)

  • Testing large amplitude versus small amplitude response

  • Testing policies entering at different points

  • Testing different patterns of behavior

  • Isolating uniqueness of equilibrium or steady state

  • Understanding forces producing equilibrium positions (Mass 1991/1981)


Summary
Summary

  • Modelers, stakeholders, problem experts, and others in the modeling process pursue validity at every step along the way.

  • We have rigorous traditions guiding model creation, formulation, exploration, and implications.

  • We have a powerful, intimidating battery of tests of model structure and behavior.

  • Model-based conclusions that make it through all this deserve the confidence of everyone in the process.


Epilog
Epilog

  • Reason is itself a matter of faith. It is an act of faith to assert that our thoughts have any relation to reality. (G.K. Chesterton)

  • I have no exquisite reason for’t, but I have reason good enough. (Sir Andrew, Twelfth Night)


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