Comment by ccppurcell

Well it only works when there is no information at all apart from the past frequency.

It's the solution to the tank problem. You know that the enemy number their tanks as they're produced. You capture a tank and know its number, N. What's the best guess about how many tanks the enemy has produced so far? As a pure mathematical model with no other details, the best guess is 2N. Of course in reality you have some ideas about how long it takes to make a tank, how many resources the enemy has etc.

Analogously you have information about the way trends develop.