title: Physical activity and premenopausal breast cancer: an examination of recall and selection bias.
abstract: BACKGROUND
  Compared with cohort studies, case-control investigations have tended to report clearer protective associations for the relationship between physical activity and premenopausal breast cancer risk.
  
  
  METHODS
  We conducted a case-control study within the Nurses' Health Study II cohort to examine whether recall or selection bias could explain the stronger protective associations. Self-reported total recreational physical activity during adulthood and over a woman's lifetime (ages 12 years to current) were assessed in 1997 before diagnosis and, again, from one to seven years after breast cancer diagnosis among the same women.
  
  
  RESULTS
  Eighty-seven percent of cases (417 of 479) and 82% of controls (390 of 474) responded. Selection bias was observed for activity during adulthood but not for activity over a woman's lifetime. Recall bias was not observed in the direction we expected: the odds ratios (ORs) for breast cancer comparing the highest versus lowest quintile of prospectively reported total activity were not significantly different than the corresponding estimates from retrospective reports (e.g., lifetime activity: prospective OR = 0.58, 95% CI: 0.37, 0.93 versus retrospective OR = 0.80; 95% CI: 0.50, 1.29).
  
  
  CONCLUSIONS
  Recall or selection bias may not have been accounted for protective associations among case-control investigations examining lifetime recreational physical activity and breast cancer. Selection bias related to recreational physical activity during adulthood and random error in the measurement of physical activity remain concerns.
authors.affiliation: Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA, USA. sonia.maruti@roche.com
authors.forename: Sonia S
authors.initials: SS
authors.lastname: Maruti
authors.forename: Walter C
authors.initials: WC
authors.lastname: Willett
authors.forename: Diane
authors.initials: D
authors.lastname: Feskanich
authors.forename: Beverly
authors.initials: B
authors.lastname: Levine
authors.forename: Bernard
authors.initials: B
authors.lastname: Rosner
authors.forename: Graham A
authors.initials: GA
authors.lastname: Colditz
mesh_terms: D000328:Adult; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000517:prevention & control*; D016022:Case-Control Studies; D015331:Cohort Studies; D005260:Female; D006801:Humans; D009043:Motor Activity; D017697:Premenopause / Q000502:physiology*; D015983:Selection Bias; D011795:Surveys and Questionnaires