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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 />

1.1 Review <strong>of</strong> 3 study designs<br />

1. Cross-sectional study:<br />

• Information on disease status (Y ) and exposure status (X) is<br />

obtained <strong>from</strong> a random sample at one time point. A snap<br />

shot <strong>of</strong> population.<br />

• A single observation <strong>of</strong> each variable <strong>of</strong> interest is measured <strong>from</strong><br />

each subject: (Yi, Xi) (i = 1, ..., n). Regression such as logistic<br />

regression (if Yi is binary) can be used to assess the<br />

association between Y and X:<br />

� �<br />

P[Yi = 1|Xi]<br />

log<br />

= β0 + β1Xi<br />

1 − P[Yi = 1|Xi]<br />

� �<br />

P[Y = 1|X = 1]/(1 − P[Y = 1|X = 1])<br />

β1 = log<br />

P[Y = 1|X = 0]/(1 − P[Y = 1|X = 0])<br />

β1 = log odds-ratio between exposure population (X = 1) and<br />

non exposure population (X = 0). β1 > 0 =⇒ exposure<br />

population has higher probability <strong>of</strong> getting disease.<br />

Graduate Summer Session in Epidemiology Slide 6

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