Loss to follow-up in a longitudinal study on aging in Spain

The objectives of this study were to describe follow-up dynamics in a longi tudinal study on aging conducted in Spain between 1993 and 1997, and to ide ntify the demographic, behavioral and health characteristics of persons who would later refuse to continue participating, move out of the area or be h ard to locate (i.e., become lost to follow-up subjects). Data from the 1993 baseline survey were used to predict the probabilities of being lost to fo llow-up in the 1995 and 1997 waves. Structural multiple logistic regression s were fitted and mean probabilities were estimated to identify patterns of loss to follow-up. After 4 years, 52% of baseline participants remained in the study, 24% had died, 17% refused to continue participating, and 8.7% w ere impossible to locate. In the multivariate analysis, advanced age and li ving alone were independent predictors of loss to follow-up, and none of th e health status variables remained significant. However, participation stat us in previous waves and the number of nonresponse items were strong indepe ndent predictors of further non-participation. Our results suggest that an attitude against participation in surveys may be an independent predictor o f losses to follow-up and efforts should be made to retain this subgroup of the population in the study. Further research on reasons why people are un willing to participate in surveys and strategies to retain people in longit udinal studies is needed. (C) 2001 Elsevier Science Inc. All rights reserve d.