The difference between a one-tailed and two-tailed test of
significance is that a one-tailed test looks for an increase or decrease in the
parameter and a two-tailed test looks for any change in the parameter
(Stockburger, n.d.). The one-tailed hypothesis-testing procedure is for a
directional hypothesis and the two-tailed hypothesis-testing is a procedure for
a nondirectional hypothesis. A one- or two-tailed t-test is determined by
whether the total area of a is placed in one tail or divided equally between
the two tails (Stockburger, n.d.). When the results are interesting only if
they turn out in a particular direction a one-tailed test is performed and if the results would be interesting in either
direction a two-tailed test is performed. In a one-tailed test the area
associated with a is in either one tail or the other, and the selection of the
tail depends which direction would be positive or negative (Stockburger, n.d.)
A two-tailed test divides a in half by placing half in the each tail, therefore
the null hypothesis is a particular value with two alternative hypotheses, one
positive and one negative (Stockburger, n.d.). The hypothesis-testing procedure
is effected in different ways depending on the choice of one- or two-tailed
tests.

Reference

Stockburger, D.W.
(n.d.). Introductory Statistics: Concepts, Models, and Applications . Retrieved
from http://www.psychstat.missouristate.edu/introbook/sbk25m.htm

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