Unknown unknowns How does Good-Turing frequency estimation - and all of the smoothing techniques that follow - fit into all of this? It’s all about leaving some room for previously unseen stuff in your probability distribution, estimating how much by looking at how many things you’ve seen only rarely. Possibly it’s analogous to having a feeling for how “crud-prone” some particular endeavour is; some things are generally pretty reliable, with some things you know you can pretty much rely on unexpected trouble and delays. “If I think of all the work I know needs to be done, this looks like a day-and-a-half job. If I think of how long similar jobs have taken in the past, two weeks”, and it ends up taking three weeks.