External validity of a population-based study on osteoporosis and fracture

Abstract

Background and purpose — Little is known about the characteristics of non-participants in epidemiological studies. We evaluated external validity by comparing fracture and mortality rate in participants and non-participants in a longitudinal study on risk factors for fracture.Methods — 1,604 randomly selected women, 75 years of age, were invited to attend a study on osteoporosis and fracture. 1,044 women attended the study (participants) and 560 women did not participate (non-participants). Fracture data for all were obtained prospectively from radiographic records. Mortality data were obtained through the population register. Mean follow-up was 13 (11–15) years. Cumulative survival was compared with the log-rank test. Fracture incidence rates per 1,000 person-years were compared with Mann-Whitney U-tests. In addition, fracture comparisons were made with the cumulative incidence function and Gray’s test.Results — 454 participants (44%) died during the follow-up, as compared to 372 of the non-participants (66%) (p Interpretation — Our findings suggest that participants had a lower mortality rate than non-participants. Distal forearm and vertebral fractures were more frequent in participants. However, the external validity for fractures in general appeared to be satisfactory.