Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2015 Jul 10;10(7):e0132110.
doi: 10.1371/journal.pone.0132110. eCollection 2015.

Detection of Outliers Due to Participants' Non-Adherence to Protocol in a Longitudinal Study of Cognitive Decline

Aline Dugravot  1 , Severine Sabia  2 , Martin J Shipley  3 , Catherine Welch  3 , Mika Kivimaki  3 , Archana Singh-Manoux  4
Affiliations

Detection of Outliers Due to Participants' Non-Adherence to Protocol in a Longitudinal Study of Cognitive Decline

Aline Dugravot et al. PLoS One. .

Abstract

Background: Participants' non adherence to protocol affects data quality. In longitudinal studies, this leads to outliers that can be present at the level of the population or the individual. The purpose of the present study is to elaborate a method for detection of outliers in a study of cognitive ageing.

Methods: In the Whitehall II study, data on a cognitive test battery have been collected in 1997-99, 2002-04, 2007-09 and 2012-13. Outliers at the 2012-13 wave were identified using a 4-step procedure: (1) identify cognitive tests with potential non-adherence to protocol, (2) choose a prediction model between a simple model with socio-demographic covariates and one that also includes health behaviours and health measures, (3) define an outlier using a studentized residual, and (4) study the impact of exclusion of outliers by estimating the effect of age and diabetes on cognitive decline.

Results: 5516 participants provided cognitive data in 2012-13. Comparisons of rates of annual decline over the first three and all four waves of data suggested outliers in three of the 5 tests. Mean residuals for the 2012-13 wave were larger for the basic compared to the more complex prediction model (all p<0.001), leading us to use the latter for the identification of outliers. Residuals greater than two standard deviation of residuals identified approximately 7% of observations as being outliers. Removal of these observations from the analyses showed that both age and diabetes had associations with cognitive decline similar to that observed with the first three waves of data; these associations were weaker or absent in non-cleaned data.

Conclusions: Identification of outliers is important as they obscure the effects of known risk factor and introduce bias in the estimates of cognitive decline. We showed that an informed approach, using the range of data collected in a longitudinal study, may be able to identify outliers.

PubMed Disclaimer

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Sample selection, Whitehall II Study.
See this image and copyright information in PMC

References

    1. Dodd S, White IR, Williamson P. (2012) Nonadherence to treatment protocol in published randomised controlled trials: a review. Trials 13: 84 10.1186/1745-6215-13-84 - DOI - PMC - PubMed
    1. Fergusson D, Aaron SD, Guyatt G, Hebert P. (2002) Post-randomisation exclusions: the intention to treat principle and excluding patients from analysis. BMJ (Clinical research ed 325: 652–654. - PMC - PubMed
    1. Grubbs FE. (1969) Procedures for detecting outlying obesrvations in samples. Technometrics 11: 1–21.
    1. Motulsky HJ, Brown RE. (2006) Detecting outliers when fitting data with nonlinear regression—a new method based on robust nonlinear regression and the false discovery rate. BMC Bioinformatics 7: 123 - PMC - PubMed
    1. McClure FD, Lee JK, Wilson DB. (2003) Validity of the percent reduction in standard deviation outlier test for screening laboratory means from a collaborative study. J AOAC Int 86: 1045–1055. - PubMed

Publication types

LinkOut - more resources