Tuesday, November 10, 2009

Predictions, Forecasts, projections and scenarios

OK, so looks like the theme for the week is why I'm not making predictions. Or rather, that I am making predictions but really I ought to be building scenarios. Audrey Coreau and colleagues have a nice paper in Ecology Letters (2009 12: 1277–1286) on "The rise of research on futures in ecology". Their essential thesis is that we need to spend more time talking about what hasn't happened yet, i.e. the future, because it will help strengthen the basics of our science. Oh, and incidentally it will help with decision making. They build on the distinctions MacCraken made between predictions, forecasts, projections and scenarios primarily emphasizing the value of qualitative scenarios for developing possible futures. They do see a role for predictive models within those scenarios, so phew, I'm not out of a job yet.

I find their definition of a projection curious: "a statement about what would happen, based on the extrapolation of past and current trends (e.g. population projections)." The way they use the term projection it is based on observed data extrapolated out into the future - assuming no change in what is presently going on. This is consistent with how Hal Caswell describes matrix projection models of populations, although those are quite different from simple extrapolation based on trend data. What makes this definition curious is that it is different from how MacCraken defines a projection: "a projection is a probabilistic statement that it is possible that something will happen in the future if certain conditions develop. " Specifically - what happens if conditions ARE NOT the same as they are now, and in particular, if conditions are affected by management options between now and then.

Hmm, so I guess I'm making projections sensu MacCracken.

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