Full description of controls is needed in studyBMJ 1996; 312 doi: https://doi.org/10.1136/bmj.312.7030.577b (Published 02 March 1996) Cite this as: BMJ 1996;312:577
- Peter Sasieni
- 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 …
Log in using your username and password
Log in through your institution
Register for a free trial to thebmj.com to receive unlimited access to all content on thebmj.com for 14 days.
Sign up for a free trial