What is the minimal detectable change (MDC)?

• A. The largest change expected due to practice.
• B. The smallest change that likely reflects true improvement beyond measurement error.
• C. The difference between two baseline measurements.
• D. The standard deviation of change scores.

Answer: (B)

The minimal detectable change is the smallest amount of change in a measurement that is unlikely to be due to measurement error, given a chosen level of confidence. It depends on the standard error of measurement (SEM) and accounts for variability across two measurements (such as baseline and follow-up). For 95% confidence, it’s calculated as 1.96 × SEM × √2. For example, if SEM is 2 units, the MDC95 is about 5.5 units, meaning a change smaller than that could just be noise, while a change larger than that suggests a real improvement with 95% confidence. This focuses on whether an observed change reflects true change in an individual, rather than just practice effects, a simple baseline difference, or the spread of change scores.