Which test is described as a planned comparison or post hoc test that permits either a one-tailed or two-tailed evaluation?

The Dunnett test is a planned comparison or post hoc test specifically designed to compare several treatment groups to a single control group. It is particularly valuable in studies where multiple hypotheses are being tested simultaneously, as it maintains control over the Type I error rate.

What distinguishes the Dunnett test in this context is its flexibility to allow for both one-tailed and two-tailed evaluations. A one-tailed test evaluates the probability of observing data more extreme in one direction (testing for an increase or decrease), while a two-tailed test assesses the probability in both directions. This adaptability makes the Dunnett test a popular choice among researchers when the primary interest lies in comparisons against a control, providing a clear framework for hypothesis testing in such scenarios.

In contrast, the other options have different focuses: the Scheffé test is used for multiple comparisons without specific control group comparisons; the Bonferroni adjustment is a method to correct for multiple comparisons, which is primarily concerned with reducing the likelihood of Type I errors; and the Tukey test is typically used for pairwise comparisons among group means but does not inherently allow for flexibility in one-tailed evaluations. Thus, the Dunnett test stands out as the most fitting option for the question posed.