Prior Probability
Prior probability is the base rate you start with before incorporating new evidence.
Definition
Prior probability is your starting probability estimate before new evidence. Priors reflect base rates and context.
Why it matters
Bayes updates depend on where you start. A strong likelihood ratio changes outcomes differently for a 5 percent prior than for a 50 percent prior.
Common pitfalls
Using 0 percent or 100 percent priors: Bayes updates break because odds are undefined.
Ignoring base rates: New information is not the whole story.