- The fourth step is to compute p, the
probability (or probability value) of
obtaining a difference between and the value specified by the null
hypothesis (zero) as large or larger than the difference obtained in
the experiment. This can be calculated using the following formula:

The degrees of freedom are N-2 where N is the sample size.

For this example,

The df are N - 2 = 100 - 2 = 98. A t table can be used to find that the two-tailed probability value for a t of 2.776 with 98 df is 0.007.