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EPID 766: Analysis of Longitudinal Data from ... - NCSU Statistics

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CHAPTER 1 Epid <strong>766</strong>, D. Zhang<br />

1.5 Why longitudinal studies?<br />

1. A longitudinal study allows us to study the change <strong>of</strong> the variable <strong>of</strong><br />

interest over time, either population level or individual level.<br />

2. A longitudinal study enables us to separately estimate the<br />

cross-sectional effect (e.g., cohort effect) and the longitudinal effect<br />

(e.g., aging effect):<br />

Given yij, age ij (j = 1, 2, · · · , ni, j = 1 is baseline). In a<br />

cross-sectional study, ni = 1 and we are forced to fit the following<br />

model<br />

yi1 = β0 + βCage i1 + ǫi1.<br />

That is, βC is the cross-sectional effect <strong>of</strong> age.<br />

With longitudinal data (ni > 1), we can entertain the model<br />

yij = β0 + βCage i1 + βL(age ij − age i1) + ǫij.<br />

Graduate Summer Session in Epidemiology Slide 22

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