[gaming_mouse]

If there true preflop raising percentage is p, then the sample percentage after N hands has a binomial distribution with parameters with mean p and standard deviation sqrt(p*(1-p)/N).

You can now apply the normal approximation to the binomial (ie, assume a normal distribution with the same mean and SD that I calculated above) to obtain confidence intervals.

For example, if you observed a PFR=.1 after 50 trials, a 95% CI for the true PFR is about [.015,.185], which is a fairly large interval.

I think in general people give too much credence to stats after for small sample size.