title: Time-dependent confounding in the study of the effects of regular physical activity in chronic obstructive pulmonary disease: an application of the marginal structural model.
abstract: OBJECTIVE
  Results from longitudinal studies about the association between physical activity and chronic obstructive pulmonary disease (COPD) may have been biased because they did not properly adjust for time-dependent confounders. Marginal structural models (MSMs) have been proposed to address this type of confounding. We sought to assess the presence of time-dependent confounding in the association between physical activity and COPD development and course by comparing risk estimates between standard statistical methods and MSMs.
  
  
  METHODS
  By using the population-based cohort Copenhagen City Heart Study, 6,568 subjects selected from the general population in 1976 were followed up until 2004 with three repeated examinations.
  
  
  RESULTS
  Moderate to high compared with low physical activity was associated with a reduced risk of developing COPD both in the standard analysis (odds ratio [OR] 0.76, p = 0.007) and in the MSM analysis (OR 0.79, p = 0.025). In the subgroup with COPD (n = 2,226), high physical activity was associated with a reduced risk of COPD admissions during follow-up (standard, incidence rate ratio, 0.74; p = 0.096; MSM, 0.68, p = 0.044), and with a reduced risk of mortality (standard, hazard ratio 0.80, p = 0.001; MSM, 0.81, p = 0.008).
  
  
  CONCLUSIONS
  These results support the previously reported associations between physical activity and reduced risk of COPD development, hospitalizations, and mortality, thereby suggesting they were not due to time-dependent confounding.
authors: Centre for Research in Environmental Epidemiology, Barcelona, Catalonia, Spain. jgarcia@creal.cat
authors: Judith
authors: J
authors: Garcia-Aymerich
authors: Peter
authors: P
authors: Lange
authors: Ignasi
authors: I
authors: Serra
authors: Peter
authors: P
authors: Schnohr
authors: Josep M
authors: JM
authors: Antó
mesh_terms: D000203:Activities of Daily Living; D015986:Confounding Factors, Epidemiologic; D015444:Exercise; D005500:Follow-Up Studies; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D008961:Models, Structural; D009043:Motor Activity; D009426:Netherlands / Q000453:epidemiology; D029424:Pulmonary Disease, Chronic Obstructive / Q000401:mortality* / Q000517:prevention & control; D011795:Surveys and Questionnaires; D016019:Survival Analysis