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Statistical assessment of MMSE scores

Whi Oranga, Nelson Hospital, Private Bag 18, Whakat 7001, New Zealand, e-mail: Alastair.Willis{at}nmhs.govt.nz

It is disappointing that the interesting study by O’Loughlin & Darley (Psychiatric Bulletin, April 2006, 30, 131–134) was let down by the use of inappropriate statistics. Since scores on the Mini-Mental State Examination (MMSE) constitute data that are ordinal in nature, it is not appropriate for the mean to be presented as a measure of central tendency. For the same reason, it is not appropriate for standard deviation to be offered as a measure of dispersion. Use of the median and interquartile range (IQR) would have been more appropriate. Similarly, use of the t-test as a test for difference between the two groups was ill considered because MMSE scores in both study populations are negatively skewed. The authors should have used a non-parametric test for difference such as the Mann–Whitney U-test.

For the record, the median MMSE score was 20 (IQR 16–24) in the 1996 sample and 22 (IQR 19–25) in the 2003 sample. Running the authors’ data through a Mann–Whitney test on Stat Crunch (available at http://www.statcrunch.com) still finds a significant difference between the two groups (P=0.0037).

This Article Full Text (PDF) Submit an eLetter Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Willis, A. Search for Related Content PubMed Articles by Willis, A.