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 even 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" wherein people feel that the same event 'obviously' belongs to two different reference classes, and that the experiments were performed in cases where the new example was highly similar to past examples (no more dissimilar to them than they were to each other). I.e., this year's Christmas shopping optimism and last year's Christmas shopping optimism are in fact much more similar than the invention of the Internet to the invention of agriculture; and if someone else feels that the invention of the Internet is more like the invention of television, Yudkowsky worries that trying to plead "But the outside view is better than the inside view!" ends up with no possible resolution except "I'm taking my reference class and going home!"