Kahneman and Tversky dubbed these two perspectives the “inside view” (looking at the individual project in its singularity) and the “outside view” (looking at a project as part of a class of projects, as “one of those”). Both are valuable. But they’re very different. Although there’s little danger that a forecaster will ignore the inside view, overlooking the outside view is routine. That’s a fatal error. To produce a reliable forecast, you need the outside view.
