• breast cancer
• abortion

Goldacre et al¹ use an impressively large database (28 616 cases and 325 456 controls) and an observed/expected ratio significantly below unity (0.83; 95% CI 0.74 to 0.93) in their record linkage study, to conclude that induced abortion “does not increase the risk of breast cancer.”

In their discussion, however, the authors acknowledge a massive deficiency—that is, that their “data on abortions are substantially incomplete because they only include women admitted to hospital (and) only include those in the care of the National Health Service (NHS)”. Considering that the majority of English abortions do not occur in NHS hospitals, most of the women in the study who did indeed have an induced abortion are probably misclassified as not having had any. The even more egregious nature of this flaw is reflected in the fact that a mere 300 cases—just over 1% of the total—are classified as having had an induced abortion. As the overall induced abortion rate in England and Wales averaged more than 1% per year during the study period (1968–1998),² it is conservatively estimated that approximately 15% of the women in the cohort underwent an induced abortion in their lifetime. Consequently, more than 90% of the women in the study cohort who underwent induced abortion were misclassified as not having an induced abortion. Therefore, the Goldacre et al dataset is wholly inapplicable to the question of an association between induced abortion and breast cancer.

Such inappropriate use of a large dataset is reminiscent of a similar report from 1982 by an Oxford group with authorship overlapping that of the present study (D Yeates).³ In the 1982 study, Vessey et al, using …