Order Effects

Order effects occur when the sequence of tasks or conditions influences results. If participants always test Design A before Design B, you cannot tell whether B scored higher because it is better or because participants were more practiced by the time they saw it.

Types of Order Effects

• Practice effects: Participants get better at the task with repetition. The second condition benefits from learned skills, regardless of which design it is
• Fatigue effects: Participants get tired, bored, or less careful over time. Later conditions suffer from reduced attention and motivation
• Carry-over effects: Exposure to the first condition changes how participants perceive or interact with the second. If you test a cluttered interface first, a clean one feels better by contrast
• Sensitization: Testing the first condition alerts participants to what you are measuring, changing how they approach subsequent conditions

Why It Matters

Order effects are especially dangerous because they masquerade as real differences between conditions. Your data will show a statistically significant result, and the result is real—but it measures the effect of sequence, not design.

The Fix

Counterbalancing is the standard solution: vary the order across participants so each condition appears equally often in each position. This does not eliminate order effects but allows you to measure and control for them in your analysis.