What does error variance refer to in research designs?

Error variance refers to the variability in the dependent variable that cannot be attributed to the independent variable being tested. It reflects the random fluctuations or "noise" that occur within a set of data and can arise from a variety of sources, including individual differences among participants, measurement error, environmental conditions, and more.

In the context of within-group designs, error variance is particularly relevant because these designs utilize the same subjects across different conditions. As a result, any inherent variability associated with those subjects can contribute to error variance. This "noise" complicates the data interpretation, as it obscures the true effect of the independent variable on the dependent variable. Understanding error variance is crucial for researchers to improve their study design and enhance the reliability of their measurements, which ultimately aids in drawing more accurate conclusions from their findings.