External validity of a population-based study on osteoporosis and fracture.
OBJECTIVE
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 < 0.001). The fracture incidence rate for any type of fracture was 43 for participants and 47 for non-participants (p = 1.0). The fracture incidence rate for typical osteoporotic fracture was 36 for participants and 39 for non-participants (p = 0.6). The corresponding values for distal forearm fracture were 11 and 7 (p = 0.002), they were 8 and 9 for proximal humerus fracture (p = 0.9), 13 and 10 for vertebral fracture (p = 0.007), 15 and 18 for hip fracture (p = 0.8), and they were 6 and 5 for pelvic fracture (p = 0.3). The cumulative incidence function confirmed the results.


CONCLUSIONS
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.
Department of Clinical Science , Intervention and Technology, Karolinska Institutet, Department of Orthopedics, Karolinska University Hospital Huddinge , Stockholm.
Axel
A
Wihlborg
Kristina
K
Åkesson
Paul
P
Gerdhem
D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D005500:Follow-Up Studies; D050723:Fractures, Bone / Q000453:epidemiology / Q000401:mortality*; D006620:Hip Fractures / Q000453:epidemiology / Q000401:mortality; D006801:Humans; D015994:Incidence; D053208:Kaplan-Meier Estimate; D008137:Longitudinal Studies; D010024:Osteoporosis / Q000453:epidemiology / Q000401:mortality*; D010384:Pelvic Bones / Q000293:injuries; D012042:Registries / Q000706:statistics & numerical data; D015203:Reproducibility of Results; D012307:Risk Factors; D012784:Shoulder Fractures / Q000453:epidemiology / Q000401:mortality; D016103:Spinal Fractures / Q000453:epidemiology / Q000401:mortality; D013548:Sweden / Q000453:epidemiology