-Scores can decrease over time due to fatigue, or increase due to learning. 
-In taste tests, a dry wine may get a higher rank if it was preceded by a dryer wine and a lower rank if preceded by a sweeter wine. 
THEREFORE: Order effects can interfere with the analysis’ ability to correctly estimate the effect of the treatment itself.

University of Michigan - Ann Arbor

An order effect is a primary disadvantage of within-subjects experimental designs. It occurs when participants' responses in various experimental conditions are systematically affected by the order in which they were exposed to those conditions. When order effects are not the direct focus of the research, they can act as problematic confounding variables that obscure the true effect of the independent variable.

Order Effect

James, G., Witten, D., Hastie, T., & Tibshirani, R. (2013). An introduction to statistical learning (Vol. 112, pp. 3-7). New York: springer.

An Introduction to Statistical Learning with Applications in R

Example of order effects

1. Randomization
2. Allowing time between treatments
3. Counterbalancing the order of treatments among others. 
4. ALTERNATIVE: use independent groups design if applicable

Methods to reduce drawbacks

A carryover effect is a specific type of order effect that occurs when a participant's behavior or response in one experimental condition is directly influenced by having been tested in a prior condition. When these effects are not the primary focus of the research, they can become problematic confounding variables, making it difficult to determine whether changes in the dependent variable are due to the independent variable or merely the lingering impact of previous conditions.

Carryover Effect

Counterbalancing is a methodological solution to the problem of order effects in within-subjects designs. It involves systematically testing different participants in different sequences of conditions. By varying the order, counterbalancing ensures that the sequence of conditions does not become a confounding variable, allowing researchers to either neutralize order effects across the study or detect them by analyzing the data separately for each specific order.

Counterbalancing

In the context of within-subjects experimental designs, what term describes the phenomenon where participants' responses are systematically altered by the sequence in which they experience the experimental conditions?

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