Abstract
Background. Attrition is a major cause of potential bias in longitudinal studies and clinical trials. Attrition rate above 20% raises concern of the reliability of the results. Few studies have looked at the factors behind attrition in follow-ups spanning decades.Methods. We analyzed attrition and associated factors of a 30-year follow-up cohort of subjects who were born with perinatal risks for neurodevelopmental disorders. Attrition rates were calculated at different stages of follow-up and differences between responders and non-responders were tested. To find combinations of variables influencing attrition and investigate their relative importance at birth, 5, 9, 16 and 30 years of follow-up we used the random forest classification.Results. Initial loss of potential participants was 13%. Attrition was 16% at five, 24% at nine, 35% at 16 and 46% at 30 years. The only group difference that emerged between responders and non-responders was in socioeconomic status (SES). The variables identified by random forest classification analysis were classified into Birth related, Development related and SES related. Variables from all these categories contributed to attrition, but SES related variables were less important than birth and development associated variables. Classification accuracy ranged between 0.74 and 0.96 depending on age.Discussion. Lower SES is linked to attrition in many studies. Our results point to the importance of the growth and development related factors in a longitudinal study. Parents’ decisions to participate depend on the characteristics of the child. The same association was also seen when the child, now grown up, decided to participate at 30 years. In addition, birth related medical variables are associated with the attrition still at the age of 30. Our results using a data mining approach suggest that attrition in longitudinal studies is influenced by complex interactions of a multitude of variables, which are not necessarily evident using other multivariate techniques.
Highlights
Bias arises if the rate of subjects dropping out from a longitudinal study or clinical trial is not random, and the variables responsible for a subject to drop out from the study are correlated with variables that are used to evaluate outcome
The aim is to analyze the causes of attrition, as well as to identify changes in the importance of factors associating to attrition at different stages of the follow-up
92 of them had been seen at an early clinical visit or child health centers during the first 40 months of life, only 38 were completely lost to follow-up (LTFU) after birth
Summary
Bias arises if the rate of subjects dropping out from a longitudinal study or clinical trial is not random, and the variables responsible for a subject to drop out from the study are correlated with variables that are used to evaluate outcome. Such bias is referred to as attrition bias. Attrition is a major cause of potential bias in longitudinal studies and clinical trials. To find combinations of variables influencing attrition and investigate their relative importance at birth, 5, 9, 16 and 30 years of follow-up we used the random forest classification. Our results using a data mining approach suggest that attrition in longitudinal studies is influenced by complex interactions of a multitude of variables, which are not necessarily evident using other multivariate techniques
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