Sir

We are writing to express our concern about the statistical methods used in the recent publication by Joober et al (2002) in Neuropsychopharmacology. These authors have used a panel of recombinant congenic (RC) mouse strains, conceptually similar to the more commonly used recombinant inbred (RI) mouse strains, to map quantitative trait loci (QTL) for prepulse inhibition (PPI). Joober et al measured PPI in 10–15 mice from each of 37 inbred strains and analyzed the effect of 625 previously defined genetic markers on this phenotype. The authors state: ‘For the QTL analysis, data from all animals, regardless of their genetic background, were entered into a one-way analysis of variance (ANOVA), where the genotype (A/J homozygous coded 1 and C57BL/6J coded 2) was considered as the grouping factor and ASR or PPI at different prepulse intensities was the dependent variable.’ The ANOVA model used by Joober et al incorrectly treats every mouse as an independent observation and thereby uses the mouse, rather than strain, as the experimental unit.

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