Assessing the degree of residual confounding: a cohort study on the association between disability pension and mortality.

ABSTRACT

Background: Previous studies showed that disability pensioners have a higher risk of premature death than others, but residual confounding has been suggested. The aim was to assess the degree of residual confounding of the association between disability pension (DP) and risk of premature death. Methods: Prospective cohort study of everyone aged 19-64 years, living in Sweden 31 December 2004 (n = 5 406 469), followed up through 2010. Mortality hazard rates over time were estimated for three groups; incident disability pensioners during 2005 from start of DP (February-December 2005), prevalent disability pensioners (January 2005 or since before), and individuals not on DP in January 2005, after standardizing populations to characteristics of the incident disability pensioners, stratified by previous hospitalization or not. If DP has no immediate effect on mortality, incident disability pensioners and those not on DP should initially have similar hazard rates, thereby, allowing assessment of the degree of residual confounding. Results: For those not previously hospitalized, the mortality hazard rate on the first DP day was 3.07 (95% CI 2.21, 4.36), 2.09 (1.78, 2.48) and 0.78 (0.73, 0.84) per thousand person-years for incident, prevalent, and non DP, respectively. Among previously hospitalized these figures were 21.67 (17.73, 26.24), 17.00 (15.76, 18.51) and 18.88 (18.14, 19.64) respectively. Hazard ratios were 1.15 (0.94, 1.40) in the strata with and 3.94 (2.78, 5.57) in the strata without, previous hospitalization comparing incident DP with non-DP. Conclusions: Substantial residual confounding was found in the association between DP and premature death among those not previously hospitalized.