BMJ 2001;322:362 ( 10 February )

Letters

Contraception in general practice before teenage pregnancy

    Inappropriate selection of cases and controls biased study
    Not all teenagers are sexually active
    Authors' reply
    Ambition may be best contraception
    Intrauterine device may not be most suitable method of contraception for teenagers

Inappropriate selection of cases and controls biased study

The first 150 words of the full text of this article appear below.

EDITOR---In their case-control study, Churchill et al found that teenagers who become pregnant have higher consultation rates at general practices than their age matched peers.1 However, their conclusions may be biased by inappropriate selection of cases and controls.

Churchill et al defined cases as all registered patients in the practices who had a recorded termination, delivery, or miscarriage resulting from conception. Since they ascertained cases from practice computer records, maternity books, and knowledge of practice staff, the cases included only patients who received such services locally and not from other districts. Controls were, however, selected from the ordered list for the practice closest in chronological age as the cases. Hence the control group included registered patients who were actually resident in other districts. The inclusion of such patients would seriously underestimate the consultation rates in the control group.

The median age of the subjects was 17 years. A significant . . . [Full text of this article]
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