I have been trying to brush up on my stats knowledge, especially in relation to Sample size determination and Statistical Power Analysis. But it seems that the more I read the more I need to read.

Anyway I found a tool called G*Power which seems to do everything I need but I am having an issue understanding Noncentrality Parameter, what is it, what does it do, what would be a suggested value etc?

The information on wikipedia etc is either incomplete or I'm not doing a very good job understanding it.

I am conducting a series of two tailed z-tests if that is any help.

p.s. Can anyone add better tags to this question?