Model choice can obscure results in longitudinal studies

Christopher H. Morrell, Larry J. Brant, Luigi Ferrucci

Research output: Contribution to journalArticlepeer-review

57 Scopus citations


Background. This article examines how different parameterizations of age and time in modeling observational longitudinal data can affect results. Methods. When individuals of different ages at study entry are considered, it becomes necessary to distinguish between longitudinal and cross-sectional differences to overcome possible selection biases. Results. Various models were fitted using data from longitudinal studies with participants with different ages and different follow-up lengths. Decomposing age into two components-age at entry into the study (first age) and the longitudinal follow-up (time) compared with considering age alone-leads to different conclusions. Conclusions. In general, models using both first age and time terms performed better, and these terms are usually necessary to correctly analyze longitudinal data.

Original languageEnglish (US)
Pages (from-to)215-222
Number of pages8
JournalJournals of Gerontology - Series A Biological Sciences and Medical Sciences
Issue number2
StatePublished - Feb 2009
Externally publishedYes


  • Mixed-effects models
  • Multilevel modeling
  • Observational study
  • Recruitment bias
  • Regression

ASJC Scopus subject areas

  • Aging
  • Geriatrics and Gerontology
  • Medicine(all)


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