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).
- © 2006 Royal College of Psychiatrists