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. 2009 Feb;64(2):215-22.
doi: 10.1093/gerona/gln024. Epub 2009 Feb 5.

Model choice can obscure results in longitudinal studies

Christopher H Morrell  1 , Larry J Brant, Luigi Ferrucci
Affiliations

Model choice can obscure results in longitudinal studies

Christopher H Morrell et al. J Gerontol A Biol Sci Med Sci. 2009 Feb.

Abstract

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.

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Figures

Figure 1.
Figure 1.
Observed (thin lines) and modeled body weights of women with first age at least 70 years’ old with no more than three repeated measurements. The thick dash-dot line is from the Age Model 1 and the thick solid lines are from the FAge and Time Model 2.
Figure 2.
Figure 2.
Observed (thin lines) and modeled cholesterol levels (mg/dL) of men with first age at least 60 years’ old with 2 to four repeated measurements. The thick dash-dot line is from the Age Model 3 and the thick solid lines are from the FAge and Time Model 4.
Figure 3.
Figure 3.
Observed (thin lines) and modeled body weights of women with two repeated measurements. The thick dash-dot line is from the Age Model 5 and the thick solid lines are from the FAge and Time Model 6.
Figure 4.
Figure 4.
Observed (thin lines) and modeled maximum systolic blood pressure (mmHg) of women aged 40 and older with at most five repeated measurements. The thick dash-dot line is from the Age Model 7 and the thick solid lines are from the FAge and Time Model 8.
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