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Abstract:
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Longitudinal studies follow N participants, and data on variables of interest are collected r more times after baseline for each participant. In some studies, the number of participants is fixed and the investigator needs to determine the minimum number of additional measurements subject to a pre-specified power constraint. In other studies, the number of times measurements are taken is fixed and the investigator needs to determine how many participants are needed to attain a fixed power. And in some studies, both N and r are free, and the investigator may choose the combination that minimizes study cost for a fixed power, or that maximizes power for a fixed cost. In a longitudinal study, the investigator must specify features of the correlation matrix that describe the relationship between repeated measures from the same person in addition to the usual design inputs. Methods previously developed in the context of clinical trials are extended to allow for exposure prevalence to vary, allowing for primary time metrics other than duration of follow-up, for time-varying exposures, and to allow for correlation between the primary time metric and exposure. Software is available to implement these methods (www.hsph.harvard.edu/faculty/spiegelman/optitxs.html).
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