A factor could have a main effect when:Group of answer choicesThe mean score at each of its levels is greater than the mean score at any level of the other factorThe marginal mean scores of the conditions making up that factor are not equalThe marginal mean scores of the conditions making up that factor are equal

A factor could have a main effect when the marginal mean scores of the conditions making up that factor are not equal.

Here's a step-by-step explanation:

1. In an experiment, a factor is an independent variable manipulated by the researcher. Each factor has two or more levels that are the specific values or conditions of the factor that are tested in the experiment.

2. The main effect of a factor refers to the effect of changing the levels of that factor while keeping all other factors constant.

3. The mean score at each level of a factor is calculated by averaging the scores of all participants in that level.

4. The marginal mean score of a factor is the mean score averaged across all levels of the other factors.

5. If the marginal mean scores of the conditions making up a factor are equal, it means that changing the levels of that factor does not have a significant effect on the dependent variable.

6. If the marginal mean scores of the conditions making up a factor are not equal, it means that changing the levels of that factor does have a significant effect on the dependent variable. This is when we can say a factor has a main effect.

So, a factor could have a main effect when the marginal mean scores of the conditions making up that factor are not equal.