Reference class forecasting
Observed to work against
- planning fallacy (Osberg and Shrauger 1986)
(Note: When people use the overloaded expression "taking the Outside view", they may mean reference class forecasting, or they may mean other completely different things.)
In short: If you want to guess how long a task will take, ask how long a similar task has taken you in the past, or how long you think someone else would expect you to take. You can apply the technique to many things, not just time prediction.
There's not always a natural reference class. In that case, you're "reasoning by analogy".
For example, if you want to invade Ukraine, ask how the US fared in Iraq before you commit to the decision. This may be perfectly good to do, but it's not strict RCF, just analogy. It's still better the more similar the analogy: the US did not border Iraq nor was it seeking to annex it, so ideally you find a more closely matching example.