title: Population versus hospital controls for case-control studies on cancers in Chinese hospitals.
abstract: BACKGROUND
  Correct control selection is crucial to the internal validity of case-control studies. Little information exists on differences between population and hospital controls in case-control studies on cancers in Chinese hospital setting.
  
  
  METHODS
  We conducted three parallel case-control studies on leukemia, breast and colorectal cancers in China between 2009 and 2010, using population and hospital controls to separately match 540 incident cases by age, gender and residency at a 1:1 ratio. Demographic and lifestyle factors were measured using a validated questionnaire in face-to-face interview. Odds ratios (ORs) and 95% confidence intervals (CIs) were obtained using conditional logistic regression analyses.
  
  
  RESULTS
  The two control groups had closely similar exposure distributions of 15 out of 16 factors, with the only exception being that hospital controls were less likely to have a BMI ≥ 25 (OR = 0.71, 95% CI: 0.54, 0.93). For exposure of green tea drinking, the adjusted ORs (95% CIs) comparing green tealeaves intake ≥ 1000 grams annually with non-drinkers were 0.51 (0.31, 0.83) and 0.21 (0.27, 0.74) for three cancers combined, 0.06 (0.01, 0.61) and 0.07 (0.01, 0.47) for breast cancer, 0.52 (0.29, 0.94) and 0.45 (0.25, 0.82) for colorectal cancer, 0.65 (0.08, 5.63) and 0.57 (0.07, 4.79) for leukemia using hospital and population controls respectively.
  
  
  CONCLUSIONS
  The study found that hospital controls were comparable with population controls for most demographic characteristics and lifestyle factors measured, but there was a slight difference between the two control groups. Hospital outpatients provide a satisfactory control group in hospital-based case-control study in the Chinese hospital setting.
authors.affiliation: School of Population Health, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.
authors.forename: Lin
authors.initials: L
authors.lastname: Li
authors.forename: Min
authors.initials: M
authors.lastname: Zhang
authors.forename: D'Arcy
authors.initials: D
authors.lastname: Holman
mesh_terms: D000368:Aged; D000428:Alcohol Drinking; D044466:Asian People / Q000706:statistics & numerical data; D001943:Breast Neoplasms / Q000208:ethnology / Q000517:prevention & control*; D016022:Case-Control Studies; D002681:China / Q000453:epidemiology; D015179:Colorectal Neoplasms / Q000208:ethnology / Q000517:prevention & control*; D004326:Drinking; D005260:Female; D006784:Hospitals, Teaching; D006801:Humans; D007297:Inpatients / Q000706:statistics & numerical data; D007407:Interviews as Topic; D007938:Leukemia / Q000208:ethnology / Q000517:prevention & control*; D008019:Life Style; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D010045:Outpatients / Q000706:statistics & numerical data; D011795:Surveys and Questionnaires; D013662:Tea
chemical_list: D013662:Tea