Statistical issues on studies of mixed longitudinal design

Mixed longitudinal designs are important study designs for many areas of medical research. Mixed longitudinal studies have several advantages over cross-sectional or pure longitudinal studies, including shorter study completion time and ability to separate time and age effects, thus are an attractive choice. Statistical methodology used in general longitudinal studies has been rapidly developing within the last few decades. Common approaches for statistical modeling in studies with mixed longitudinal designs have been the linear mixed-effects model incorporating an age or time effect. The general linear mixed-effects model is considered an appropriate choice to analyze repeated measurements data in longitudinal studies. However, common use of linear mixed-effects model on mixed longitudinal studies often incorporates age as the only random-effect but fails to take into consideration the cohort effect in conducting statistical inferences on age-related trajectories of outcome measurements. We believe special attention should be paid to cohort effects when analyzing data in mixed longitudinal designs with multiple overlapping cohorts. Thus, this has become an important statistical issue to address. ^ This research aims to address statistical issues related to mixed longitudinal studies. The proposed study examined the existing statistical analysis methods for the mixed longitudinal designs and developed an alternative analytic method to incorporate effects from multiple overlapping cohorts as well as from different aged subjects. The proposed study used simulation to evaluate the performance of the proposed analytic method by comparing it with the commonly-used model. Finally, the study applied the proposed analytic method to the data collected by an existing study Project HeartBeat!, which had been evaluated using traditional analytic techniques. Project HeartBeat! is a longitudinal study of cardiovascular disease (CVD) risk factors in childhood and adolescence using a mixed longitudinal design. The proposed model was used to evaluate four blood lipids adjusting for age, gender, race/ethnicity, and endocrine hormones. The result of this dissertation suggest the proposed analytic model could be a more flexible and reliable choice than the traditional model in terms of fitting data to provide more accurate estimates in mixed longitudinal studies. Conceptually, the proposed model described in this study has useful features, including consideration of effects from multiple overlapping cohorts, and is an attractive approach for analyzing data in mixed longitudinal design studies.^

Comparison of alternative methods of contact tracing in a syphilis control program

The purpose of this study was to compare the relative effectiveness of alternative methods of tracing named contacts of syphilis patients. A total of 236 contacts, identified by patients in two City of Houston Department of Health and Human Services clinics during the period April 1 through July 31, 1987, were studied. After contacts were grouped by sex and age, the proportion brought to examination by each of three methods, and by a combination of methods, was determined for each subgroup.^ The study found that 78.4% of all the 236 named sex contacts reported were located and brought to examination by the various methods of contact tracing and that 21.6% were missed. Of the 185 contacts examined, a combination of methods identified 47.7% of the cases, telephone contact, 28.6%, field contact, 16.9%, and patient referral, 11.8%.^ Of the 236 contacts reported, males made up 56.8% and females 43.2%. Contact tracing was more successful among females, with 81.4% of the 102 named female contacts, as compared to 76.1% of the 134 named male contacts being brought to examination. It is not known whether equal efforts were exerted in the follow-up of both male and female contacts. In both female and male subgroups, a combination of methods brought over 40% of sex contacts to examination. Telephone contact among females yielded 27.7% of the cases and field contact 18.1%, whereas in males, telephone contact identified 29.4% of the cases and field contact 15.7%. Patient referral was the least productive method in both sex groups, locating 12.8% in males as compared to 10.8% in females.^ On an age specific basis, a combination of methods was the most effective method in the 15-39 age group, whereas telephone contact was most effective in the 40-44 age group, and field contact in the 50-54 age group. Of all the methods of contact tracing, patient referral was the least productive in most age groups.^ Future studies of contact tracing should incorporate several important variables which were not examined in this study. ^