Modelers Can Often Obtain the Desired Result
(p. A13) After earning a master's degree in environmental engineering in 1982, I spent most of the next 10 years building large-scale environmental computer models. My first job was as a consultant to the Environmental Protection Agency. I was hired to build a model to assess the impact of its Construction Grants Program, a nationwide effort in the 1970s and 1980s to upgrade sewer-treatment plants.
The computer model was huge--it analyzed every river, sewer treatment plant and drinking-water intake (the places in rivers where municipalities draw their water) in the country. I'll spare you the details, but the model showed huge gains from the program as water quality improved dramatically. By the late 1980s, however, any gains from upgrading sewer treatments would be offset by the additional pollution load coming from people who moved from on-site septic tanks to public sewers, which dump the waste into rivers. Basically the model said we had hit the point of diminishing returns.
When I presented the results to the EPA official in charge, he said that I should go back and "sharpen my pencil." I did. I reviewed assumptions, tweaked coefficients and recalibrated data. But when I reran everything the numbers didn't change much. At our next meeting he told me to run the numbers again.
After three iterations I finally blurted out, "What number are you looking for?" He didn't miss a beat: He told me that he needed to show $2 billion of benefits to get the program renewed. I finally turned enough knobs to get the answer he wanted, and everyone was happy.
. . .
There are no exact values for the coefficients in models such as these. There are only ranges of potential values. By moving a bunch of these parameters to one side or the other you can usually get very different results, often (surprise) in line with your initial beliefs.
For the full commentary, see:
ROBERT J. CAPRARA. "OPINION; Confessions of a Computer Modeler; Any model, including those predicting climate doom, can be tweaked to yield a desired result. I should know." The Wall Street Journal (Weds., July 9, 2014): A13.
(Note: ellipsis added.)
(Note: the online version of the commentary has the date July 8, 2014.)