An Inside View on a topic involves making predictions based on your understanding of the details of the process. An Outside View involves ignoring these details and using an estimate based on a class of roughly similar previous
For example, someone working on a project may estimate that they can reasonably get 20% of it done per day, so they will get it done in five days (inside view). Or they might consider that all of their previous projects were completed just before the deadline, so since the deadline for this project is in 30 days,
that's when it will get done (outside view). Another term for taking an outside view is reference class forecasting.
The planning example is discussed in The Planning Fallacy. Possible limitations and problems with using the outside view are discussed in The Outside
View's Domain and "Outside View" as Conversation-Halter. Model Combination and Adjustment discusses the implications of there usually existing multiple different outside views.
I did 1500 piece jigsaw puzzle of fireworks, my first jigsaw in at least ten years.
Several times I had the strong impression that I had carefully eliminated every possible place a piece could go, or every possible piece that could go in a place. I was very tempted to conclude that many pieces were missing, or that the box had extra pieces from another puzzle. This wasn’t impossible – the puzzle was an open box a relative had done before. And the alternative seemed humiliating.
But I allowed a very different part of my mind, using different considerations, to overrule this judgment; so many extra or missing pieces seemed unlikely.
And in the end there was only one missing and no extra pieces. I recall a similar experience when I was learning to program. I would carefully check my program and find no errors, and then when my program wouldn’t run I was tempted to suspect compiler or hardware errors. Of course the problem was almost always my fault. -- Robin Hanson, Beware the Inside View