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In simple terms, a base rate is history: what happened last time, and the time before, in situations like the one you're facing. Of every hire into this role, how many stayed? Of every project this size, how many landed on time?
That history exists in every industry, and usually inside your own organisation already. A leavers' log is a base rate for retention, and a record of won and lost tenders is one for your next bid. Nobody has to call it a base rate for it to be one.
Because it's built from many past cases, one striking success or one disaster barely moves it. The details of the case in front of you still count, and the history gives them something to be measured against.
It works alongside a premortem. The premortem asks how this particular plan could fail, and the base rate tells you how often plans like it already have.
The practical tell is a team that can describe this candidate or plan in detail but can't say how the last few like it turned out. That gap is where the base rate belongs.
Twelve weeks into his new job as an assistant manager, David resigned, and the reason he gave was the one the business's own history had predicted: travel. He lived thirty miles away, an hour's drive into the rising sun every morning and the setting sun every evening, so each eight-hour shift became a ten-hour day. I didn't recruit him, and I couldn't see why anyone would: the business had watched enough long-distance hires leave early to stop taking them on, and that record of past leavers was the base rate. The pressure to fill the post and David's confidence at interview were enough to look past it. After he left, the company made it a rule to recruit locally.