Can a transposition table cause search instability

Question

I'm writing a chess engine and recently added a transposition table.

When running a few tests, I found that although the search still returned the same best move, the value of the move (how good it is for the maximizing player) fluctuated.

Is this normal behavior for a transposition table? I remember reading that a transposition table can cause search instability. Is this what that means? So is this a normal occurrence or a serious bug in my code?

Answer 1

Null-move can cause search instability, but it can also make your program play like it's on steroids (it is a very good thing when implemented properly). My guess is that you probably have a bug somewhere in your null-move code. It only takes a very small change to have things blow up in your face. Maybe you have a ''<'' where there should be a ''>''. The shallow null-move search is done with a zero-width window around alpha. It could prevent further exploration of a node for a particular value of alpha, but not another.