What is the best way to determine the existence of a main effect?

The best way to determine the existence of a main effect is by comparing the marginal means between groups. This approach allows researchers to evaluate how different levels of an independent variable affect the dependent variable across the conditions being tested.

Marginal means represent the average scores for each level of an independent variable, calculated while ignoring the other variables in the analysis. By examining these means, you can assess whether there are significant differences between the groups, which indicates a main effect. If the marginal means show substantial variation, it suggests the factor being studied does impact the outcome in a meaningful way.

This method is preferred because it provides a clear and organized way to visualize the influence of the independent variable without the noise of individual variability or the complexity of interactions with other variables. Analyzing just total scores, individual scores, or focusing on variability within groups does not directly indicate whether a main effect is present, as these methods do not effectively isolate the effects of the independent variable.