Percentiles please: The case for expressing neuropsychological test scores and accompanying confidence limits as percentile ranks

Crawford, John R. and Garthwaite, Paul H. (2009). Percentiles please: The case for expressing neuropsychological test scores and accompanying confidence limits as percentile ranks. The Clinical Neuropsychologist, 23(2) pp. 193–204.

DOI: https://doi.org/10.1080/13854040801968450

Abstract

Many commentators on neuropsychological assessment stress the disadvantages of expressing test scores in the form of percentile ranks. As a result, there is a danger of losing sight of the fundamentals: percentile ranks express scores in a form that is of greater relevance to the neuropsychologist than any alternative metric because they tell us directly how common or uncommon such scores are in the normative population. We advocate that, in addition to expressing scores on a standard metric, neuropsychologists should also routinely record the percentile rank of all test scores so that the latter are available when attempting to reach a formulation. In addition, it is argued that the current practice of expressing confidence limits on test scores on a standard score metric should be supplemented with confidence limits expressed as percentile ranks, because the latter provide a more direct and tangible indication of the uncertainty surrounding an observed score. Computer programs accompany this paper and can be used to obtain percentile rank confidence limits for Index scores (and FSIQs) on the WAIS-III or WISC-IV (these can be downloaded from the following web page: http://www.abdn.ac.uk/~psy086/dept/PRCLME.htm).

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