What is a main effect?

What is a main effect?

• A. The effect of one factor across all its levels, irrespective of any other factors
• B. The effect of all factors combined
• C. The interaction between factors
• D. A post-hoc test result

Answer: (A)

In factorial designs, a main effect is about the influence of one factor on the outcome, averaged over the levels of the other factor(s). It asks what change in the dependent variable you would see when you move that single factor from one level to another, ignoring which level the other factors are at. For example, if you have caffeine level (low vs high) and sleep condition (short vs long), the main effect of caffeine is the average difference in sleep outcomes between low and high caffeine when you average over the short and long sleep conditions. This is distinct from the interaction, which looks at whether the effect of one factor changes depending on the level of another factor, and from an overall combined effect or post-hoc results.

In factorial designs, a main effect is about the influence of one factor on the outcome, averaged over the levels of the other factor(s). It asks what change in the dependent variable you would see when you move that single factor from one level to another, ignoring which level the other factors are at. For example, if you have caffeine level (low vs high) and sleep condition (short vs long), the main effect of caffeine is the average difference in sleep outcomes between low and high caffeine when you average over the short and long sleep conditions. This is distinct from the interaction, which looks at whether the effect of one factor changes depending on the level of another factor, and from an overall combined effect or post-hoc results.