Results cannot be generalised

EDITOR,—I have been trying to understand the method of analysis used by Stewart Irvine and colleagues to assess sperm quality in their birth cohort study.1 I think that they have not adequately taken account of possible confounding factors and have reached conclusions that are untenable.

The subjects were volunteer donors recruited in different ways–from antenatal parent craft groups, undergraduates, and personal contacts of existing donors. Given the diverse nature of those recruited, the authors must have analysed the …