Statistics Notes

Missing data

BMJ 2007; 334 doi: http://dx.doi.org/10.1136/bmj.38977.682025.2C (Published 22 February 2007)
Cite this as: BMJ 2007;334:424

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  1. Douglas G Altman, professor of statistics in medicine1,
  2. J Martin Bland, professor of health statistics2
  1. 1Cancer Research UK/NHS Centre for Statistics in Medicine, Oxford OX2 6UD
  2. 2Department of Health Sciences, University of York, York YO10 5DD
  1. Correspondence to: Professor Altman doug.altman{at}cancer.org.uk

    Almost all studies have some missing observations. Yet textbooks and software commonly assume that data are complete, and the topic of how to handle missing data is not often discussed outside statistics journals.

    There are many types of missing data and different reasons for data being missing. Both issues affect the analysis. Some examples are:

    • (1) In a postal questionnaire survey not all the selected individuals respond;

    • (2) In a randomised trial some patients are lost to follow up before the end of the study;

    • (3) In a multicentre study some centres do not measure a particular variable;

    • (4) In a study in which patients are assessed frequently some data are missing at some time points for unknown reasons;

    • (5) Occasional data values for a variable are missing because some equipment failed;

    • (6) Some laboratory samples are lost in transit or technically unsatisfactory;

    • (7) In a magnetic resonance imaging study some very obese patients are excluded …

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