Letters

Contraception in general practice before teenage pregnancy

BMJ 2001; 322 doi: https://doi.org/10.1136/bmj.322.7282.362/a (Published 10 February 2001) Cite this as: BMJ 2001;322:362

Inappropriate selection of cases and controls biased study

  1. Wai-Ching Leung (W.C.Leung@ncl.ac.uk), senior registrar in public health medicine, epidemiology and public health
  1. Newcastle General Hospital, Newcastle upon Tyne NE4 6BE
  2. Department of Epidemiology and Public Health, Newcastle Medical School, Newcastle upon Tyne NE2 4HH
  3. Division of General Practice, University of Nottingham Medical School, Queen's Medical Centre, Nottingham NG7 2UH
  4. Department of Geography, University of Nottingham, Nottingham NG7 2RD
  5. Department of Obstetrics and Gynaecology
  6. School of Nursing Queen's Medical Centre, Nottingham, NG7 2UH
  7. Oxford OX1 2NA
  8. Oxford OX1 4LJ
  9. FPA Health, Sydney, NSW 2131, Australia

    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 proportion of the control subjects were in further education and well over a third of the over 18s were in higher education.2 Hence the selection bias is likely to be considerable and could account for the findings in the study.

    Footnotes

    • Competing interests None declared.

    References

    1. 1.
    2. 2.

    Not all teenagers are sexually active

    1. K Duffield, stage 3 medical student,
    2. P K Josen, stage 3 medical student,
    3. E Low, stage 3 medical student,
    4. K Teare, stage 3 medical student,
    5. E Wray, stage 3 medical student
    1. Newcastle General Hospital, Newcastle upon Tyne NE4 6BE
    2. Department of Epidemiology and Public Health, Newcastle Medical School, Newcastle upon Tyne NE2 4HH
    3. Division of General Practice, University of Nottingham Medical School, Queen's Medical Centre, Nottingham NG7 2UH
    4. Department of Geography, University of Nottingham, Nottingham NG7 2RD
    5. Department of Obstetrics and Gynaecology
    6. School of Nursing Queen's Medical Centre, Nottingham, NG7 2UH
    7. Oxford OX1 2NA
    8. Oxford OX1 4LJ
    9. FPA Health, Sydney, NSW 2131, Australia

      EDITOR—Churchill et al in their study dispel the myth that teenagers do not consult their general practitioners about contraception.1 They also conclude, however, that teenagers who become pregnant have higher consultation rates than their age matched peers, most of the difference owing to consultation for contraception.

      We do not believe that this claim is justified, as the potential confounder of sexual activity was not controlled for. While the case group (pregnant teenagers) must be sexually active, the same cannot be said of the age matched controls. It …

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