I was recently asked to examine some single-case study data and ended up using the method described by Mueser, Yarnold, and Foy (1991). In order to use this method, your data must comprise a minimum of four assessment points at equally spaced intervals (e.g., baseline, 12-weeks, 24-weeks, and 36-weeks).

This method is based on classical test theory and the steps involved are as follows:

- Calculate ipsative z-scores for the participant
- Calculate the 1-lag autocorrelation factor
- Calculate the critical difference (CD)
- For each assessment point, we calculate the difference between the corresponding z-score and the z-score at baseline.
- If the absolute value of this difference score is greater than the CD value, then it is statistically significant.

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