Format

Send to:

Choose Destination
See comment in PubMed Commons below
Contemp Clin Trials. 2011 May;32(3):412-7. doi: 10.1016/j.cct.2011.01.002. Epub 2011 Jan 15.

How many measurements for time-averaged differences in repeated measurement studies?

Author information

  • 1Department of Clinical Sciences, UT Southwestern Medical Center, Dallas, TX 75390, USA.

Abstract

In many studies, investigators have perceived the number of repeated measurements as a fixed design characteristic. However, the number of repeated measurements is a design choice that can be informed by statistical considerations. In this paper, we investigate how the number of repeated measurements affects the required sample size in longitudinal studies with scheduled assessment times and a fixed total duration. It is shown that the required sample size always decreases as the number of measurements per subject increases under the compound symmetry (CS) correlation. The magnitude of sample size reduction, however, quickly shrinks to less than 5% when the number of measurements per subject increases beyond 4. We then reveal a counterintuitive property of the AR(1) correlation structure, under which making additional measurements from each subject might increase the sample size requirement. This observation suggests that practitioners should be cautious about assuming the AR(1) model in repeated measurements studies, whether in experimental design or in data analysis. Finally, we show that by introducing measurement error into the AR(1) model, the counterintuitive behavior disappears. That is, additional measurements per subject result in reduced sample sizes.

Copyright © 2011 Elsevier Inc. All rights reserved.

PMID:
21241827
[PubMed - indexed for MEDLINE]
PMCID:
PMC3070039
Free PMC Article
PubMed Commons home

PubMed Commons

0 comments
How to join PubMed Commons

    Supplemental Content

    Full text links

    Icon for Elsevier Science Icon for PubMed Central
    Loading ...
    Write to the Help Desk