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. Such repeatedmea-suresdataarecorrelatedwithin subjectsandthus secondimportant outcomethatis commonlymeasuredin a longitudinalstudyis thetimeuntil a key clinicalevent such as diseaserecurrenceor LONGITUDINALDATA ANALYSISA nalysisof event timeendpoints is thefocusofsurvivalanalysiswhich iscoveredin Chapter ?
1.2 Exploratory Data Analysis Exploratory analysis of longitudinal data seeks to discover patterns of sys-tematic variation across groups of patients, as well as aspects of random variation that distinguish individual patients. 1.2.1 Group means over time When scienti c interest is in the average response over time, summary statis-
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