Access to the full text of this article requires a subscription or payment. Please log in or subscribe below.

  1. Peter Sasieni
  1. Research scientist Department of Mathematics, Statistics, and Epidemiology, Imperial Cancer Research Fund, PO Box 123, London WC2A 3PX

    EDITOR,—It is not uncommon to select controls from two sources because of concerns about the suitability of any one source. Superficial analysis of the data presented by Michael A Lewis and colleagues shows that hospital controls who use oral contraceptives are roughly three times less likely than community controls to use third generation pills (table 1).1 Naively, one might conclude that third generation pills are a general tonic, particularly in comparison with second generation pills: women who use them are …

    Access to the full text of this article requires a subscription or payment

    Subscribe

    Article access

    Article access for 1 day

    Purchase this article for £20 $30 €32*

    The PDF version can be downloaded as your personal record

    * Prices do not include VAT

    Rapid responses

    THIS WEEK'S POLL

    Read related article

    See previous polls

    BMJ most popular