Apologies for the nature of my post but I'm a relative beginner to statistical tests. What I would like to know, is if I have two sets of data (as I'm a microbiologist, the data sets are likely to be bacterial colony size measurements on two different agar plate types), why would I choose to do the t-test over the f-test and vice versa?

From my understanding, the t-test is used when the standard deviations of the two sets of data are not significantly different from one another, and the f-test is used when the SDs are significantly different from one another. But my question would be: how do we define "signficantly different from one another" without performing some kind of analysis? Any help would be appreciated. Go easy!

Thanks,

Craig