Final answer:
You must calculate weightings when performing a planned contrast primarily to adjust for unequal sample sizes, ensuring fair comparisons between groups and maintaining the integrity of the statistical test.
Step-by-step explanation:
When performing a planned contrast in hypothesis testing, it is important to calculate weightings primarily to adjust for unequal sample sizes. This process allows for fair comparisons between groups by ensuring that group differences that are due to the independent variable are not confounded by differences in group sizes. Furthermore, if the assumption of equal variance is violated in an ANOVA, calculating weightings can also help in adjusting for this issue, thus maintaining the integrity of the test results.
Planned contrasts are a type of hypothesis test that focus on comparing specific means or groups identified a priori rather than conducting multiple post hoc comparisons. When calculating a planned contrast, weightings are applied to each group mean so that the resulting statistic correctly reflects the hypothesis being tested.
The effectiveness of experimental controls, such as ensuring all groups differ by only one independent variable, is critical for valid comparative analyses, but adjusting group weightings is the specific method to account for sample size differences in a contrast analysis.