Student's t distribution

$mean=\mu$ $variance=(\frac{c}{c-2})\sigma^2$

parameter c is degrees of freedom. The variance approaches the one of the normal distribution for large c.

$t=\frac{\bar{X}-\mu}{S_X/\sqrt{n}}$

where ${S_X}^2 = \frac{1}{n-1}\sum{(X_i-\bar{X})^2}$ is the standard deviation of the sample.

Given some precalculated values from Wikipedia statistics tables, we can find the associated probability.