Kaezarhg

Been playing for some time now, and while trying to make up an all-psi squad (I'm a perfectionist, sue me) I noticed that as you go through it gets harder - or at least it seems so - to get a psi player. I think I'm getting more or less 1 psi every 10 test cycles. Is that right? Anybody knows the formula that gives the chance of a psi soldier?