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Model validity and quality: Concepts, methods and tools Yaman Barlas Boğaziçi University Industrial Engineering Department 34342 Bebek Istanbul, Turkey.

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Presentation on theme: "Model validity and quality: Concepts, methods and tools Yaman Barlas Boğaziçi University Industrial Engineering Department 34342 Bebek Istanbul, Turkey."— Presentation transcript:

1 Model validity and quality: Concepts, methods and tools Yaman Barlas Boğaziçi University Industrial Engineering Department Bebek Istanbul, Turkey SESDYN Group:

2 Conceptual and Philosophical Foundations Model Validity and Types of Models –Statistical Forecasting models (black box) –Descriptive Policy models (transparent) Philosophical Aspects - Philosophy of Science - Logical Empiricim and Absolute Truth - Conversational justification & relative truth (‘purpose’) - Statistical significance testing (Andersen, D.F. 1980, Meadows, D. H. 1980, Barlas and Carpenter 1990, and Barlas 1996)

3 Two aspects of model validity Structure Validity –Primary importance –Special place in System Dynamics Behavior Validity –Role in system dynamics –The special type of behavior validity in system dynamics –Ex ante versus ex post prediction (Forrester and Senge 1980, Barlas 1996 and 1989)

4 Overall Nature and Selected Tests of Formal Model Validation

5 Logical Sequence of Formal Steps of Model Validation

6 Validity (Quality) ‘Built-in’ vs. ‘Tested’ (Inspected) Problem ID and purpose Time unit and horizon Explicit decision: Is the model discrete or continuous? Perform DT tests (verfication) if continuous Dynamic hypothesis (main stocks, loops and reference behavior) All variables & parameters with explainable meanings All equations with explainable meanings Units and consistency! Use the established principles of good equation writing Use established (generic) formulation structures as appropriate Start with SMALL models (does NOT mean SIMPLE!) Embellish gradually, by adding one structure at a time and testing End with small models! (parsimony)

7 Structure Validity (Simulation Verification) Direct Structure Tests –Crucial, yet highly qualitative and informal –Distributed through the entire modeling methodology Indirect Structure Tests (Structure-oriented behavior) –Crucial and partly quantitative and formal –Tool: SiS software

8 Indirect Structure Testing Software: SiS Based on automated dynamic pattern recognition Extreme condition pattern testing Also in parameter calibration and policy design (Kanar and Barlas 1999; Barlas and Bog 2005)

9 Indirect Structure Testing Software (SiS) Basic Dynamic Patterns

10 Indirect Structure Testing Software (SiS) List of dynamic behavior pattern classes

11 Software Implementation Our Software (SiS) Main ISTS Algorithm Simulati on Software Integrator General Picture of the Processes in Validity Testing mode General Picture of the Processes in “Parameter Calibration” mode

12 Sample Model Used with SiS

13 Validity Testing with Default Parameters Simulation Output (with default base parameters) Likelihood Values of simulation behavior correctly classified as the GR2DB pattern

14 Validity Testing by Setting Parameters Fig1 : Simulation Output (with base parameters) Fig2 : Simulation Output (with changed parameters) Likelihood Values of simulation behavior in Fig2 compared to the NEXGR pattern

15 Parameter Calibration with Specified Pattern The ranges and number of values tried for each parameter Simulation Output (with base parameters)

16 Result of the Parameter Calibration  Best parameter set is 41  Best Likelihood Result: Best Parameter Set:  1. advertising effectiveness: 0.25  2. customer sales effectiveness: 6.0  3. sales size: 1.0 Simulation Output as Desired (after automated parameter calibration)

17 Parameter Calibration with Input Data A view of the SiS interface during parameter calibration

18 Result of the Parameter Calibration  Best parameter set is 21  Best Likelihood Result: Best Parameter Set:  1. advertising effectiveness: 5.0  2. customer sales effectiveness: 0.0 Fig1 : Simulation Output (with base parameters) Fig2 : Simulation Output (after parameter calibration to match the input pattern)

19 Behavior Validity Two types of patterns –Steady state –Transient Major pattern components –Trend, periods, amplitudes,...

20 Behavior Validity Testing Software: BTS II

21

22 BTS II Tools Trend Regression Modely(t) = a + b * t a: b:

23 BTS II Tools Moments Moment Calculations # Of Data Points: 100 1st Moment (Mean): nd Moment (Variance):

24 BTS II Tools Autocorrelation

25 BTS II Tools Autocorrelation Test

26 BTS II Tools Spectral Density Function dominant period1: 20Value : dominant period2: 8Value :

27 BTS II Tools Cross correlation Max CrossCorrelation: at lag 0

28 BTS II Tools Amplitude Estimation Modely(t) = a + b * sin ( 2 * π * t / period + c ) a : b : c : Amplitude Estimate:

29 BTS II Tools Discrepancy Coefficient # Of Data Points100 U: U U U

30 BTS II Tools Trend in Amplitude

31 constant phase angle trend of amplitude: const of amplitude : slope of amplitude :

32 Uses of BTS II and SiS in Model Analysis Analysis: Understanding the dynamic properties of the model BTS II can assist in quantifying, measuring and assessing dynamic pattern components SiS can assist in deeper structural analysis (related to qualitative pattern modes)

33 Uses of BTS II and SiS in Policy Design BTS II can assist in numerical performance improvement policies SiS can assist in more structural dynamic pattern improvement Parameter calibration can be extended to cover automated policy design

34 Implementation Issues More tools User friendliness More thorough (field) testing of the tools Better integration with simulation software...

35 Policy Implementation Issues Validity of the policy recommendation (Robustness, timing, duration, transition...) Finally, ‘validity of the implementation’ itself –Validated model means just a reliable laboratory; implementation validity does not automatically follow; it is a whole area in itself

36 Concluding Observations Validity as a process, rather than an outcome Continuous (prolonged) validity testing Validation, analysis and policy design all integrated From validity towards quality Quality ‘built-in versus inspected-in’ Group model building Testing by interactive gaming

37 References Akkermans, H.A., and J.A.M. Vennix “Clients' opinions on group model-building: an exploratory study.” System Dynamics Review 13(1): 3-31 Andersen, D.F “How Differences in Analytic Paradigms Can Lead to Differences in Policy Decisions.” In Elements of the System Dynamics Method, ed. J. Randers. Portland, OR: Productivity Press Andersen, D.F., G.P. Richardson and J.A.M. Vennix “Group model building: adding more science to the craft.” System Dynamics Review. 13(2): Ansoff, H.I and D.P. Slevin “An Appreciation of Industrial Dynamics.” Management Science 14: Barlas Y, Suat Boğ Automated dynamic pattern testing, parameter calibration and policy improvement. Proceedings of international system dynamics conference. (CD). Albany, NY, USA Barlas Y ”System Dynamics: Systemic Feedback Modeling for Policy Analysis” in Knowledge for Sustainable Development - An Insight into the Encyclopedia of Life Support Systems Vol.1, UNESCO-Eolss Publishers, Paris, France, Oxford, UK. pp Barlas, Y “Formal Aspects of Model Validity and Validation in System Dynamics”, System Dynamics Review, Vol.12, no.3, pp Barlas, Y. and A. Erdem “Output Behavior Validation in System Dynamics Simulation.” In Proceedings of the European Simulation Symposium (Istanbul, Turkey) Barlas, Y Comments on “On the Very Idea of a System Dynamics Model of Kuhnian Science.” System Dynamics Review 8(1): Barlas, Y “An Autocorrelation Function Test For Output Validation.” Simulation 55(1):7-16. Barlas, Y. and S. Carpenter “Philosophical Roots of Model Validation: Two Paradigms.” System Dynamics Review 6(2): Barlas, Y. 1989a. “Multiple Tests for Validation of System Dynamics Type of Simulation Models.” European Journal of Operational Research 42(1): Barlas, Y. 1989b. “Tests of Model Behavior That Can Detect Structural Flaws: Demonstration With Simulation Experiments.” In Computer-Based Management of Complex Systems: International System Dynamics Conference. P.M.Milling and E.O.K.Zahn, eds. Berlin: Springer-Verlag. Barlas, Y Validation of System Dynamics Models With a Sequential Procedure Involving Multiple Quantitative Methods. Unpublished Ph.D. Dissertation. Georgia Institute of Technology. Atlanta, GA

38 Bell, J.A and M.F. Bell “System Dynamics and Scientific Method.” In Elements of the System Dynamics Method, ed. J. Randers. Portland, OR: Productivity Press Carson, E.R.and R.L.Flood “Model Validation: Philosophy, Methodology and Examples.” Trans Inst MC.12(4): Coyle, G. And D. Exelby “The validation of commercial system dynamics models.” System Dynamics Review 16(1): Dangerfield, B “The System Dynamics Modeling Process and DYSMAP2.” In Modeling For Learning Organizations. Morecroft, J.D.W and J.D. Sterman, eds. Portland, OR: Productivity Press Eberlein, R.L and D.W. Peterson “Understanding Models with VENSIM.” In Modeling For Learning Organizations. Morecroft, J.D.W and J.D. Sterman, eds. Portland, OR: Productivity Press European Journal of Operational Research Special Issue on Model Validation. 66(2). Forrester J.W. and P.M.Senge “Tests For Building Confidence in System Dynamics Models.” In System Dynamics. Legasto, A.A., J.W. Forrester and J.M. Lyneis, eds. Amsterdam: North-Holland Forrester, J.W., G.W. Low and N.J. Mass “The Debate on World Dynamics: A Response to Nordhaus.” Policy Sciences 5: Forrester, J.W Confidence in Models of Social Behavior with Emphasis on System Dynamics Models. System Dynamics Group Working Paper. Sloan School of Management, MIT, Cambridge, MA. Forrester, J.W “A Response to Ansoff and Slevin.” Management Science 14: Forrester, J.W Industrial Dynamics. Portland, OR: Productivity Press Graham, A.K., J.D.W. Morecroft, P.M. Senge and J.D. Sterman “ Model-supported Case Studies in Management Education.” In Modeling For Learning Organizations. Morecroft, J.D.W and J.D. Sterman, eds. Portland, OR: Productivity Press Kanar K and Y. Barlas "A Dynamic Pattern-oriented Test for Model Validation”, Proceedings of 4th Systems Science European Congress, Valencia, Spain, Sept. 1999, pp Kleijnen, J.P.C “Verification and Validation of Simulation Models.” European Journal of Operational Research 82: Lane, D.C “The Folding Star: A Comparative Reframing and Extension of Validity Concepts in System Dynamics.” In Proceedings of International System Dynamics Conference (Tokyo, Japan). Vol.1: Lane, D.C “Rerum cognoscere causas: Part I - How do the ideas of system dynamics relate to traditional social theories and the voluntarism/determinism debate?” System Dynamics Review 17(2):

39 Lane, D.C “Rerum cognoscere causas: Part II - Opportunities generated by the agency/structure debate and suggestions for clarifying the social theoretic position of system dynamics.” System Dynamics Review 17(4): Lyneis, J.M., K.S. Reichelt and T. Sjoblom “Professional DYNAMO: Simulation Software to Facilitate Management Learning and Decision Making.” In Modeling For Learning Organizations. Morecroft, J.D.W and J.D. Sterman, eds. Portland, OR: Productivity Press Mass, N. and P.M. Senge “Alternative Tests for Selecting Model Variables.” In Elements of the System Dynamics Method, ed. J. Randers. Portland, OR: Productivity Press Meadows, D “Gaming to Implement System Dynamics Models.” In Computer-Based Management of Complex Systems: International System Dynamics Conference. P.M.Milling and E.O.K.Zahn, eds. Berlin: Springer-Verlag. Meadows, D. H “The Unavoidable A Priori.” In Elements of the System Dynamics Method, ed. J. Randers. Portland, OR: Productivity Press Miller, R.G Simultaneous Statistical Inference. New York, N.Y: Springer-Verlag Mitroff, I “Fundamental Issues in the Simulation of Human Behavior.” Management Science. 15: Morecroft, J.D.W. and J.D. Sterman, eds Modeling For Learning Organizations. Portland, OR: Productivity Press Morrison, D.E. and R.E. Henkel, eds The Significance Test Controversy. Chicago, IL: Aldine Press Nordhaus, W.D “World Dynamics: Measurement Without Data.” Economic Journal 83: Naylor, T.H. and J.M. Finger “Verification of Computer Simulation Models.” Management Science. 14: Oral, M. and O. Kettani “The Facets of the Modeling and Validation Process in Operations Research.” European Journal of Operational Research 66(2): Peterson, D.W. and R.L. Eberlein “Reality Check: A Bridge Between Systems Thinking and System Dynamics.” System Dynamics Review 10(2-3): Radzicki, M.J Comments on “On the Very Idea of a System Dynamics Model of Kuhnian Science.” System Dynamics Review 8(1): Radzicki, M.J “Methodologia Oeconomiae et Systematis Dynamis.” System Dynamics Review 6(2): Richardson, G.P “What Are We Publishing? A View from the Editor’s Desk.” System Dynamics Review 7(1): Richardson, G.P. and A.L.Pugh III Introduction To System Dynamics Modeling With DYNAMO. Portland, OR: Productivity Press.

40 Roberts, N., D. Andersen, R. Deal, M. Garet, W. Shaffer Introduction to Computer Simulation: A System Dynamics Approach. Portland, OR: Productivity Press Rouwette, E.A.J.A., J.A.M. Vennix and T. Mullekom “Group model building effectiveness: a review of assessment studies.” System Dynamics Review 18(1): 5-45 Saysel A.K and Y Barlas. “Model simplification and validation with indirect structure validity tests” System Dynamics Review, Vol. 22, No.3, 2006; Scholl, G.J “Benchmarking the System Dynamics Community: Research Results.” System Dynamics Review 11(2): Schruben, L.W “Establishing the Credibility of Simulations.” Simulation. 34(3): Senge, P.M Statistical Estimation of Feedback Models. Simulation. 28: Schlesinger, S. et al “Terminology for Model Credibility.” Simulation. 32(3): Sterman, J.D Comments on “On the Very Idea of a System Dynamics Model of Kuhnian Science.” System Dynamics Review 8(1): Sterman, J.D “Modeling Managerial Behavior: Misperceptions of Feedback in a Dynamic Decision Making Experiment.” Management Science 35(3): Sterman, J.D “Testing Behavioral Simulation Models by Direct Experiment.” Management Science 33(12): Sterman, J.D “The Growth of Knowledge: Testing a Theory of Scientific Revolutions with a Formal Model.” Technological Forecasting and Social Change 28(2): Sterman, J. D Appropriate Summary Statistics for Evaluating the Historical Fit of System Dynamics Models. Dynamica. 10(2): Vennix, J.A.M “Group model-building: tackling messy problems.” System Dynamics Review. 15(4): Vennix, J.A.M., H.A. Akkermans and E.A.J.A. Rouwette “Group model-building to facilitate organizational change: an exploratory study.” System Dynamics Review. 12(1): Wittenberg, J “On the Very Idea of a System Dynamics Model of Kuhnian Science.” System Dynamics Review 8(1): Wright, R.D “Validating Dynamic Models: An Evaluation of Tests of Predictive Power.” In Proceedings of Summer Computer Simulation Conference Yücel, G. and Y. Barlas Automated parameter specification in dynamic feedback models based on behavior pattern features. System Dynamics Review, Vol. 27, No.2, pp Zellner, A Comment on Forrester’s “Information Sources for Modeling the National Economy.” Journal of the American Statistical Association 75:


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