Statistical methods for single-case studies in neuropsychology: comparing the slope of a patient's regression line with those of a control sample

Performance on some neuropsychological tests is best expressed as the slope of a regression line. Examples include the quantification of performance on tests designed to assess the accuracy of time estimation or distance estimation. The present paper presents methods for comparing a patient's performance with a control or normative sample when performance is expressed as slope. The methods test if there is a significant difference between a patient's slope and those obtained from controls, yield an estimate of the abnormality of the patient's slope, and provide confidence limits on the level of abnormality. The methods can be used with control samples of any size and will therefore be of particular relevance to single-case researchers. A method for comparing the difference between a patient's scores on two measures with the differences observed in controls is also described (one or both measures can be slopes). The methods require only summary statistics (rather than the raw data from the normative or control sample); it is hoped that this feature will encourage the development of norms for tasks that use slopes to quantify performance. Worked examples of the statistical methods are provided using neuropsychological data and a computer program (for PCs) that implements the methods is described and made available.