Answer:
Order effect
Explanation:
Order effect refers to a common phenomenon which involves different arrangement patterns on a subject topic and the only way to prevent this is by always giving the same arrangement or treatment.
April has participants in her study complete part A, followed by B, C, and then D. After the data analysis she notices that level D was always different from the rest of the levels. This is a result of a different arrangement pattern or treatment and it clearly explains the order effect.