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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 />

If we have only one observation Yi for each subject <strong>from</strong> a sample <strong>of</strong><br />

n patients, then we can’t separate σ2 e and σ2 b . Although we can use<br />

data Y1, Y2, ..., Yn to make inference on µ, we can’t make any<br />

inference on σ2 b .<br />

However, if we have repeated (or longitudinal) measurements Yij <strong>of</strong><br />

blood pressure for each subjects, then<br />

Yij = bi + eij.<br />

Now, it is possible to make inference about all quantities µ, σ 2 b and<br />

σ 2 e.<br />

5. A longitudinal study provides more evidence for possible causal<br />

interpretation.<br />

Graduate Summer Session in Epidemiology Slide 24

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