Transcription of Chapter 1 Longitudinal Data Analysis
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Themostcommonmedicalresearch designsis a \pre-post"studyinwhich a singlebaselinehealthstatusmeasurement is obtained,aninterven-tionis administered,anda singlefollow-upmeasurement is theoutcomemeasurement canbe as-sociatedwiththechangein example,if somesubjectsaregivenplacebo whileothersaregivenanactive drug,thetwogroupscanbe comparedto seeif thechangein theoutcomeis di erent forthosesubjectswhoareactivelytreatedas comparedto viewed as thesimplestformof a prospective nition: Alongitudinalstudyrefersto aninvestigationwherepartici-pant outcomesandpossiblytreatments or exposuresarecollectedat longitudinalstudygenerallyyieldsmultiple or \repeated"measurementsoneach example,HIVpatients may be followedover timeandmonthlymeasuressuch as CD4counts,or viralloadarecollectedto charac-terizeimmunestatusanddiseaseburde nrespectively.
Analysis of an environmental exposure requires speci cation of a lag between the day of exposure and the resulting e ect. In the air pollution literature short lags of 0 to 2 days are commonly used (Samet et al. 2000; Yu et al. 2000). For both the evaluation of treatment and exposure to environmental pollution the scienti c questions focus on the
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