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Noonan

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Fashion, apparel, textile, merchandising, garments

Fashion, apparel, textile, merchandising, garments

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  1. Dumb Supermodels? On the catwalk between social and physical models Douglas S. Noonan School of Public Policy Georgia Institute of Technology
  2. On models and “supermodels” <ul><li>The topic: </li></ul><ul><ul><li>nature of models, and </li></ul></ul><ul><ul><li>models of nature </li></ul></ul><ul><li>From the perspective of an economist and policy analyst…. </li></ul>
  3. Some basic questions <ul><li>What is the air quality in a particular place, time? The climate? </li></ul><ul><li>What was the air quality in a particular place, time? The climate? </li></ul><ul><li>What will be the air quality in particular place, time? The climate? </li></ul><ul><li>What are people doing right now? </li></ul><ul><li>What were they doing? What will they do? </li></ul>
  4. What we don’t know <ul><li>We have surprisingly few “facts” or direct empirical observations. </li></ul><ul><ul><li>Most of our information, our knowledge of the world actually takes the form of estimates </li></ul></ul><ul><ul><ul><li>I don’t know what temperature it is in the hallway, but I can guess. </li></ul></ul></ul><ul><ul><ul><li>Based on a model that combines: </li></ul></ul></ul><ul><ul><ul><ul><li>theory </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>knowledge of (or assumptions about) constraints and forces in the system that produce temperature </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>other data or measures </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>“ nearby” (spatially, temporally) values </li></ul></ul></ul></ul></ul><ul><ul><ul><li>Ultimately, I rely on an estimate of the phenomenon, which derives from a model of it </li></ul></ul></ul>
  5. What we don’t know <ul><li>There is a lot we don’t know. </li></ul><ul><li>There is a lot that we estimate – using models </li></ul><ul><ul><li>We estimate </li></ul></ul><ul><ul><ul><li>Atlanta’s population and AQI yesterday </li></ul></ul></ul><ul><ul><ul><li>the composition and behavior of Atlanta’s auto fleet </li></ul></ul></ul><ul><ul><ul><li>the concentration of CO emitted from a tailpipe of a car driven past a remote sensor </li></ul></ul></ul><ul><ul><ul><li>its contribution to local air quality </li></ul></ul></ul><ul><ul><ul><li>etc. </li></ul></ul></ul>
  6. Some interesting questions <ul><li>What are the determinants of local air quality? Of climate change? </li></ul><ul><li>What are the impacts of changes in local air quality? Of climate change? </li></ul><ul><li>What is the optimal change in local air quality? In the climate? </li></ul><ul><li>How can we design policy mechanisms to improve local air quality? To mitigate climate change? </li></ul>
  7. The nature of models <ul><li>These big questions involve big, complex systems </li></ul><ul><ul><li>Some simplification required for tractability </li></ul></ul><ul><ul><li>Some simplification desirable for generalization </li></ul></ul><ul><li>So, we undertake to develop a model … a simplified representation of reality that captures, we hope, the essential elements and that explains the phenomena of interest. </li></ul><ul><ul><li>How do we do? </li></ul></ul>
  8. Some prominent models <ul><li>Models of: </li></ul><ul><ul><ul><ul><ul><li>at various scales, resolution (spatial, temporal) </li></ul></ul></ul></ul></ul><ul><ul><li>weather </li></ul></ul><ul><ul><li>air quality </li></ul></ul><ul><ul><li>climate </li></ul></ul><ul><ul><li>transportation </li></ul></ul><ul><ul><li>agricultural, industrial production </li></ul></ul><ul><ul><li>innovation </li></ul></ul><ul><ul><li>(public) health </li></ul></ul><ul><ul><li>demographics </li></ul></ul>
  9. Why model? <ul><li>It seems like an obvious question, especially to most modelers </li></ul><ul><ul><li>basic research </li></ul></ul><ul><ul><li>commercial interests </li></ul></ul><ul><ul><li>policy relevance </li></ul></ul><ul><ul><li>some other “public interest”… </li></ul></ul><ul><li>More practically, models that explain phenomena also get used in other ways </li></ul><ul><ul><li>for estimating values </li></ul></ul><ul><ul><li>for forecasting </li></ul></ul><ul><ul><ul><li>the results used pervasively for decision-making by individuals, firms, public agencies, policymakers, etc. </li></ul></ul></ul>
  10. Example model <ul><li>Suppose that we are interested in modeling the effects of “air quality action days” </li></ul><ul><ul><li>effects on what? </li></ul></ul><ul><ul><ul><li>Health of those exposed </li></ul></ul></ul><ul><ul><ul><li>Emitters’ behavior </li></ul></ul></ul><ul><ul><ul><li>Others’ behavior </li></ul></ul></ul><ul><ul><ul><li>NAAQS attainment status </li></ul></ul></ul><ul><ul><ul><li>Air quality realized </li></ul></ul></ul>
  11. Example model <ul><li>Suppose that we are interested in modeling the effects of “air quality action days” </li></ul><ul><ul><li>effects on what? </li></ul></ul><ul><ul><ul><li>Health of those exposed </li></ul></ul></ul><ul><ul><ul><li>Emitters’ behavior </li></ul></ul></ul><ul><ul><ul><li>Others’ behavior </li></ul></ul></ul><ul><ul><ul><li>NAAQS attainment status </li></ul></ul></ul><ul><ul><ul><li>Air quality realized </li></ul></ul></ul>
  12. Here be Supermodels <ul><li>We might thus imagine a “supermodel” that incorporates multiple models </li></ul><ul><ul><li>Atmospheric models </li></ul></ul><ul><ul><ul><ul><ul><li>mixing and movement of airborne chemicals </li></ul></ul></ul></ul></ul><ul><ul><li>Meteorological models </li></ul></ul><ul><ul><ul><ul><ul><li>local weather variations </li></ul></ul></ul></ul></ul><ul><ul><li>Economic models </li></ul></ul><ul><ul><ul><ul><ul><li>industrial emissions, (energy) supply models </li></ul></ul></ul></ul></ul><ul><ul><li>Transportation models </li></ul></ul><ul><ul><ul><ul><ul><li>emissions quantity, location, timing </li></ul></ul></ul></ul></ul><ul><ul><li>Other models </li></ul></ul><ul><ul><ul><li>psychology, epidemiology, etc.? </li></ul></ul></ul>
  13. Endogeneity <ul><li>An essential modeling question is the extent to which influences from one system/model depend on another </li></ul><ul><ul><li>Are emissions exogenous? Is weather exogenous? Etc. </li></ul></ul><ul><li>A supermodel might well include values or estimates from multiple models </li></ul><ul><ul><li>But do those estimates depend on each other? </li></ul></ul>
  14. “Smart” Supermodels Social models (economic, transportation, etc.) Physical models (atmospheric, meteorological, etc.)
  15. Here be Supermodels <ul><li>We might thus imagine a “supermodel” that incorporates multiple models </li></ul><ul><ul><li>Atmospheric models </li></ul></ul><ul><ul><ul><ul><ul><li>mixing and movement of airborne chemicals </li></ul></ul></ul></ul></ul><ul><ul><li>Meteorological models </li></ul></ul><ul><ul><ul><ul><ul><li>local weather variations </li></ul></ul></ul></ul></ul><ul><ul><li>Economic models </li></ul></ul><ul><ul><ul><ul><ul><li>industrial emissions, (energy) supply models </li></ul></ul></ul></ul></ul><ul><ul><li>Transportation models </li></ul></ul><ul><ul><ul><ul><ul><li>emissions quantity, location, timing </li></ul></ul></ul></ul></ul><ul><ul><li>Other models </li></ul></ul><ul><ul><ul><li>psychology, epidemiology, etc.? </li></ul></ul></ul>
  16. Dumb Supermodels <ul><li>A “dumb supermodel” might be thought of as a model that incorporates multiple models independently </li></ul><ul><ul><li>e.g., T = T(Q, W, A) A = A(Q) </li></ul></ul><ul><ul><ul><ul><ul><li>So, we could rewrite it just as: T=T(Q,W,A(Q)) </li></ul></ul></ul></ul></ul><ul><ul><li>But what about a “smart supermodel” that endogenizes air quality? </li></ul></ul>
  17. Smart Supermodels <ul><li>A “smart supermodel” might be thought of as a model implicitly defined by multiple interdependent models </li></ul><ul><ul><li>e.g., T = T(Q, W, A) A = A(Q) +  Q = Q(W, T) W = W </li></ul></ul><ul><ul><li>In this case, inputs to the model are seen to depend on the output… </li></ul></ul>
  18. Air Quality Alert Impacts <ul><li>A dumb supermodel could estimate the impacts of A on T </li></ul><ul><ul><ul><li>and thus on Q (one of the policy objectives) </li></ul></ul></ul><ul><ul><li>It would take A and Q as exogenous to T </li></ul></ul><ul><li>A smart supermodel could also estimate the impacts of A on T </li></ul><ul><ul><li>It would simultaneously estimate a system with endogenous A, Q, T, and other phenomena </li></ul></ul><ul><ul><ul><li>finding any exogeneity, a “natural experiment”, or something to identify the system poses the big research challenge </li></ul></ul></ul>
  19. Air Quality Alert Impacts <ul><li>In the Atlanta context, we have daily variation in predicted and actual ozone concentrations </li></ul>
  20. Air Quality Alert Impacts <ul><li>In the Atlanta context, we have daily variation in predicted and actual ozone concentrations </li></ul><ul><li>The predictions (“smog alerts”) aim to affect behavior </li></ul><ul><ul><li>Reduce (or re-time) emission-causing behavior like driving autos </li></ul></ul><ul><ul><li>Enable people to avoid exposure </li></ul></ul><ul><li>Similar programs in ~250 cities in US </li></ul>
  21. Air Quality Alert Impacts <ul><li>Atlanta (and other cities) include these voluntary programs in their SIPs </li></ul><ul><li>What affect do these programs have </li></ul><ul><ul><li>on behavior? </li></ul></ul><ul><ul><li>on actual air quality? </li></ul></ul><ul><li>Henry & Gordon at GSU looked into it, and found large effects in the 1998 </li></ul>
  22. Air Quality Alert Impacts <ul><li>I combine 2001 data on: </li></ul><ul><ul><li>travel (from ARC’s household travel diary), </li></ul></ul><ul><ul><li>weather (actuals and forecasts from NWS), </li></ul></ul><ul><ul><li>air quality (actuals and forecasts for EPD). </li></ul></ul><ul><li>Trip behavior estimated based on traveler and environmental characteristics. </li></ul><ul><ul><li>Weather affects both behavior and air quality </li></ul></ul>
  23. Implications and Impacts <ul><li>Ozone levels and forecasts poor predictors of travel </li></ul><ul><ul><li>No O3 effect on household’s number of driving trips </li></ul></ul><ul><ul><li>Driving trip length falls in O3 shocks; unrelated to O3 alerts </li></ul></ul><ul><ul><li>O3 alerts have positive & insig. effect on household-miles-driven </li></ul></ul><ul><ul><li>Higher O3 forecasts associated with earlier departure times (~3 min./ppb) </li></ul></ul><ul><ul><li>O3 alerts (actual or forecast) associated with later departure times (~34 min.) </li></ul></ul><ul><ul><li>Employees of firms with alternative commute perks never took those modes on alert days </li></ul></ul><ul><ul><li>Elderly never biked/walked on alert days </li></ul></ul>
  24. Histogram for departure time, all days
  25. Histogram for departure time, red-alert days
  26. Implications and Impacts <ul><li>Ozone levels and forecasts poor predictors of travel </li></ul><ul><ul><li>No O3 effect on household’s number of driving trips </li></ul></ul><ul><ul><li>Driving trip length falls in O3 shocks; unrelated to O3 alerts </li></ul></ul><ul><ul><li>O3 alerts have positive & insig. effect on household-miles-driven </li></ul></ul><ul><ul><li>Higher O3 forecasts associated with earlier departure times (~3 min./ppb) </li></ul></ul><ul><ul><li>O3 alerts (actual or forecast) associated with later departure times (~34 min.) </li></ul></ul><ul><ul><li>Employees of firms with alternative commute perks never took those modes on alert days </li></ul></ul><ul><ul><li>Elderly never biked/walked on alert days </li></ul></ul>
  27. Outdoor activities/exercise <ul><li>Logits on trip involving outdoor activity </li></ul><ul><ul><li>Negative effect of forecasted prob. of rain </li></ul></ul><ul><ul><li>Negative effect of forecasted ozone levels </li></ul></ul><ul><ul><li>No effect of ozone alerts </li></ul></ul>
  28. Air Quality Alert Impacts <ul><li>Questions remain: </li></ul><ul><ul><li>Does the model generating “smog alerts” depend on behavioral changes in response to other determinants of ozone levels? Behavioral changes in response to the alerts themselves? </li></ul></ul>
  29. Smart or Dumb Supermodels <ul><li>In practice, how well supermodels integrate physical and social systems remains an open question… …even when the original purpose for the model is to explain the impact of one system on the other </li></ul><ul><ul><li>Commonly, effects are modeled as one-sided, where environmental systems respond to human influences or vice versa </li></ul></ul>
  30. Smart Supermodels (“on the catwalk”) <ul><li>Making estimates (projections, forecasts) using Supermodels smartly involves: </li></ul><ul><ul><li>modeling environmental systems </li></ul></ul><ul><ul><li>modeling social systems </li></ul></ul><ul><ul><li>modeling the interdependence between them </li></ul></ul><ul><ul><ul><li>These feedback mechanisms are often vastly more complex and misunderstood than the outcomes (e.g., environmental quality, social welfare) the we ultimately care about </li></ul></ul></ul>
  31. Other Supermodels <ul><li>Air pollution and epidemiology </li></ul><ul><li>Global climate change </li></ul><ul><li>Environmental Kuznets Curves </li></ul>
  32. Other Supermodels <ul><li>Air pollution and epidemiology </li></ul><ul><ul><li>Health = H(Enviro) </li></ul></ul><ul><ul><li>vs. </li></ul></ul><ul><ul><li>Health = H(Enviro, Indiv) </li></ul></ul><ul><ul><li>Enviro = E(Indiv, Health) </li></ul></ul><ul><ul><li>Indiv = Indiv </li></ul></ul>Classic dose-response curve Here, environmental quality is not exogenous; possibly chosen based on individual traits
  33. Other Supermodels <ul><li>Air pollution and epidemiology </li></ul><ul><li>Global climate change </li></ul><ul><ul><li>Climate = C(economic activity) </li></ul></ul><ul><ul><li>vs. </li></ul></ul><ul><ul><li>Climate = C(economic activity) </li></ul></ul><ul><ul><li>Economic activity = E(Climate) </li></ul></ul>
  34. Other Supermodels <ul><li>Air pollution and epidemiology </li></ul><ul><li>Global climate change </li></ul><ul><li>Environmental Kuznets Curves </li></ul>
  35. Adaptation and Information <ul><li>People adapt to changing environments </li></ul><ul><ul><li>Cleaner air should attract more residents </li></ul></ul><ul><ul><ul><li>Prices for housing rises, wages fall </li></ul></ul></ul><ul><ul><ul><li>Some people move in, some move out </li></ul></ul></ul><ul><ul><ul><ul><li>These are nonrandom groups </li></ul></ul></ul></ul><ul><ul><li>This different population does different things </li></ul></ul><ul><ul><ul><li>Emissions change </li></ul></ul></ul><ul><ul><ul><li>Air quality changes </li></ul></ul></ul>
  36. On the Catwalk <ul><li>The mechanism linking the physical and social systems is crucial, even if it is not the object of study itself </li></ul><ul><ul><li>What feedback mechanisms are there? </li></ul></ul><ul><ul><li>How do the models capture them? </li></ul></ul><ul><ul><li>More attention need be paid to explicitly modeling adaptations of natural, social systems to one another </li></ul></ul><ul><ul><ul><li>Malthus wasn’t the first modeler to make the mistake </li></ul></ul></ul>
  37. On the Catwalk <ul><li>Ceteris paribus assumptions, while convenient, often contradict the premise of the modeling exercise </li></ul><ul><ul><li>“Given current status/trends in X, then we forecast Y will have this [fate]” </li></ul></ul><ul><ul><li>We give the forecast to inform our efforts to change X or somehow improve the [fate] </li></ul></ul><ul><ul><ul><li>The forecast is premised on an adaptation that violates the “given current status/trends” assumption </li></ul></ul></ul>
  38. Dumb Supermodels and Trying to be Wrong <ul><li>Many dumb supermodels do not incorporate adaptation or feedbacks, leading to stark estimates or forecasts </li></ul><ul><ul><li>Strategically, very effective </li></ul></ul><ul><ul><ul><li>A model that shows “what bad would happen (in the absence of X)” is a good way to motivate X </li></ul></ul></ul><ul><ul><li>Yet if supermodels’ estimates are to be used in decision-making, these dumb supermodels will be wrong and biased </li></ul></ul><ul><ul><ul><li>Bad choices, policy result </li></ul></ul></ul><ul><ul><ul><li>Credibility fades </li></ul></ul></ul>

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