Newsgroups: comp.risks X-issue: 3.61 Date: Sat, 20 Sep 86 16:11:42 edt From: harvard!wanginst!infinet!rhorn@seismo.CSS.GOV (Rob Horn) To: risks-request@CSL.SRI.COM Subject: Simulation risk One kind of risk that I have not seen discussed here is the problems posed by using computer simulation models that are not adequate. In particular I am refering to situations where due to either insufficient computer resources, or insufficient mathematical analysis, the really accurate model results are not available. Usually more primitive, inaccurate model results are available and being used by the ideologues on both sides of an issue. This places the responsible scientists and engineers in a difficult situation. How do you say ``I don't know yet'' and how do you deal with making recommendations in the absence of adequate information. I can think of two such situations that have major public decision-making impact. The first is the ``nuclear winter'' situation. I remember many years ago reading the sensitivity analysis of the one-dimensional and two-dimensional climate models to solar input. They were hyper-sensitive, with variations on the order of measurement error causing massive climate change. It was not until recently (1982) that the vectorized Climate Model was analyzed and shown to be reasonably well behaved. And even it has some contentious approximations. This model requires 15 hours on a CRAY-1 to analyze one situation for one season. When the nuclear winter stories came out I had my doubts. Where did these people find a solid month (12 seasons x 4(?) test cases) of CRAY time? Had they used one of the hyper-sensitive 1 or 2-dimensional models. What would the accurate models find? And how should I respond when I knew that it would probably be a year or more before that much CRAY time and post- simulation analysis could be finished? (Fortunately I only had to handle party conversation with people who knew that I had worked in that field.) The same kind of problem occured in the ozone layer issues during the mid 70's. The more accurate model had two extremely severe problems: 1) it was unconditionally unstable when phrased as a finite difference problem or exceedingly temperamental when phrased as an implicit differencing problem. 2) It involved solving extremely stiff differential equations. In this case the official answer given was ``we don't know. It will take several years of mathematical research effort to make this problem tractable. The real answer is anyone's guess. The published model answers are meaningless.'' A truthful answer but of little value to decision makers. (There was a brute force throw-computers-at-it solution. Estimated run-time on a CRAY was about 1,000 years per simulated year. Easier to wait and see what happened.) How often are we placed in a situation where the inaccurate computer simulation is available, but the accurate simulation unavailable? What is an appropriate way to deal with this problem? Rob Horn UUCP: ...{decvax, seismo!harvard}!wanginst!infinet!rhorn Snail: Infinet, 40 High St., North Andover, MA

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