The purpose of planned comparisons is which of the following?

• A. To test all possible group differences after data collection.
• B. To adjust p-values for multiple testing.
• C. To test a priori hypotheses about group differences.
• D. To estimate population parameters.

Answer: (C)

Planned comparisons focus on testing hypotheses you specify before looking at the data. In a study with several groups, you might have theoretical reasons to expect certain Differences and not others. By planning specific contrasts in advance, you test those predicted differences directly rather than examining every possible difference after seeing the data. This gives you more statistical power for the hypotheses you care about and helps control the error rate for the planned set of tests. The idea of testing all possible group differences after data collection describes a post hoc approach, which is not what planned comparisons are for. Adjusting p-values for multiple testing is a feature of post hoc procedures, whereas planned comparisons center on testing pre-specified hypotheses. Estimating population parameters is not the primary goal of planned comparisons, which are about comparing means to test those hypotheses.