# One- and Two-Tailed Tests (2 of 4)

A probability computed considering differences in both directions
is called a "two-tailed" probability. The name makes sense since both
tails of the sampling distribution are considered.

There are
situations in which an experimenter is concerned only with
differences in one direction. For example, an experimenter may be
concerned with whether or not μ

_{1} - μ

_{2} is greater
than zero. However, if
μ

_{1} - μ

_{2} is not
greater than zero, the experimenter may not care whether it equals
zero or is less than zero. For instance, if a new drug treatment is
developed, the main issue is whether or not it is better than a
placebo. If the treatment is not better than a placebo, then it will
not be used. It does not really matter whether or not it is worse
than the placebo.

When only one direction is of concern to an
experimenter, then a "one-tailed" test can be performed. If an
experimenter were only concerned with whether or not μ

_{1} - μ

_{2} is
greater than zero, then the one-tailed test would involve calculating the
probability of obtaining a statistic as great or greater than the one
obtained in the experiment.