[flight2q]

[ QUOTE ]
What is a priori distribution?

[/ QUOTE ]An a priori distribution is an assumed or known probability distribution for the object of study in advance of looking at the data.

For example, tonight I hear a wolf howling. About 10% of nights this is so. I don't say, okay I'm 90% convinced that there's a full moon tonight (even without a model of the relationship between howling and full moons!).

But suppose I knew that wolf always howls when there is a full moon, and sometimes at other times. But I still don't know how likely it is that there is a full moon tonight. Now I add an a priori distribution that moon is full 1/28th of the time. Now I can deduce a posteriori that there is a 35.7% probability that the moon is full based on my knowledge of wolves, moons, hearing the howling, and assuming I have no other information such as whether I heard howling last night.