The last step in significance
testing involves calculating
the probability that a statistic would differ
as much or more from the parameter specified
in the null hypothesis as does the statistics obtained in the experiment.
A probability computed considering differences in only one direction such as the statistic is larger than the parameter is called a one-tailed probability. For example, if a parameter is 0 and the statistic is 12, a one-tailed probability (the positive tail) would be the probability of a statistic being ≥ to 12. Compare with two-tailed probability which would be the probability of being either ≤ -12 or ≥12.