Data were collected as part of a longitudinal study in Japan of adults with chronic illnesses who joined a program to learn self-management skills [13
]. The program comprised group-discussion sessions with two lay leaders, and there was one session each week for six consecutive weeks. The program was open to women and men equally. Participation in the program and in this research were voluntary. This study was approved by the Research Ethics Committee of the Graduate School of Medicine at the University of Tokyo.
Before the first group-discussion session, informed consent was obtained in writing and data were collected with a questionnaire. This baseline questionnaire asked about age, schooling, marital status, and diagnoses. It also had questions asking about health status, health-related behaviors, and psychological factors, including self-efficacy for self-management of chronic disease.
The program was organized and administered by the Japan Chronic Disease Self-Management Association. That association invited researchers from the University of Tokyo (faculty and postgraduate students) to the first session for each discussion group. Thus, the people in some of the groups were introduced to one of the researchers, who explained the study and then distributed the informed-consent form and the baseline questionnaire. After distributing them, the researcher waited, and then collected them. The first baseline questionnaires were completed in August 2006. However, because the sessions were held outside Tokyo, this required at least one postgraduate student to be away for at least two days every time a new discussion group was organized, so the research team decided (in February 2008) to try sending the informed-consent forms and baseline questionnaires by postal mail instead. Those documents were then sent (together with a self-addressed post-paid envelope) and returned by postal mail about two weeks before the first group-discussion session. Researchers were not among the leaders of the discussion groups, and thus the people who received the informed-consent forms and baseline questionnaires by postal mail had no face-to-face contact with a researcher.
Follow-up questionnaires were sent by postal mail 3, 6, and 12 months later. A self-addressed post-paid envelope was included. If a follow-up questionnaire was not returned within two weeks, a reminder postcard was sent. The postcards were preprinted and then signed by hand (by MJP). A person was classified as completely lost to follow-up if none of the three follow-up questionnaires had been returned by two months after the last one was sent. The last follow-up questionnaires were sent in December 2010.
We tested the hypothesis that complete non-participation in follow-up was directly related to another form of non-participation: absence, defined as the number of group-discussion sessions not attended. Also, because some of the baseline questionnaires were distributed in person by a researcher and others were sent by postal mail, we were able to test the hypothesis that complete loss to follow-up was more common among people who had not had face-to-face contact with one of the researchers.
In addition, we quantified associations with predictors of attrition or missing data that have been studied previously, though in other countries and in different clinical contexts: self-efficacy [14
], multimorbidity [16
], diagnosis of depression [17
], sex [16
], age [16
], schooling [16
], and marital status [16
], and we examined associations with other diagnoses (allergic disease, asthma, cancer, cardiovascular disease, connective tissue disease, diabetes, fibromyalgia syndrome, pulmonary disease, rheumatic disease, and vascular disease).
To analyze the data we used IBM SPSS version 19. As preliminary bivariate screening tests, for each categorical variable we used Fisher's exact test and for each continuous variable we used the unpaired t-test or the Mann-Whitney U test. Then, using the predictors with P
< 0.05 from those tests we also did logistic regression analyses, including multivariate analysis (P
values are listed in the supplementary table in Additional file 1
Predictors can also be evaluated in terms of their sensitivity and specificity, and the area under the receiver operating characteristic (ROC) curve. Those indices are commonly used to evaluate predictive models [24
], and they have been used previously to evaluate predictors of attrition in longitudinal studies of health-related interventions and educational programs [26
]. They also provide a basis for estimating how many of the people who would otherwise be lost to follow-up could instead be identified beforehand if a predictor is used. The maximum values of sensitivity, specificity, and the area under the ROC curve are all 1, and, in general, better predictors have higher values. Introductions to this topic can be found on the Internet [33
] and in reference 34 [34
], and more details about the use of sensitivity, specificity, and ROC curves in the analysis of predictors can be found in references 24 [24
], 25 [25
], and 35 [35