Color-coding in SMSS just got me here... I needed to write STDEVP, but as soon as STDEV turned the right color, I went on to write the next expression, and didn't get the results I expected. Bad Marcelo. 🙂
In any case, the rule is fairly straight forward to remember (and you can always see the list of aggregate functions here to reassure yourself): for a function that acts on a sample, a "P" suffix indicates that it acts on the population instead.
And you can always look it up on Live if you can't remember which one you should use.