Taking the outside view means using an estimate based on a class of roughly similar previous cases, rather than trying to visualize the details of a process. For example, estimating the completion time of a programming project based on how long similar projects have taken in the past, rather than by drawing up a graph of tasks and their expected completion times. The planning fallacy is that people tend to be hugely optimistic when visualizing the details of a case, and become more optimistic as they visualize more details.
It is controversial what exact lesson to draw from such experiments. Robin Hanson argues that this implies that, in futurism, forecasts should be made by trying to find a reference class of similar cases, rather than by trying to visualize outcomes. Eliezer Yudkowsky responds that this leads to "reference class tennis", and that the experiments were performed in cases where the new example was extremely similar to past examples (no more dissimilar to them than they were to each other).