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Statistics notes: Calculating correlation coefficients with repeated observations: Part 1—correlation within subjects

BMJ 1995; 310 doi: (Published 18 February 1995) Cite this as: BMJ 1995;310:446
  1. J Martin Bland, reader in medical statisticsa,
  2. Douglas G Altman, headb
  1. a Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
  2. b Medical Statistics Laboratory, Imperial Cancer Research Fund, PO Box 123, London WC2A 3PX
  1. Correspondence to: Dr Bland.

    In an earlier Statistics Note1 we commented on the analysis of paired data where there is more than one observation per subject, as shown in table I. We pointed out that it could be highly misleading to analyse such data by combining repeated observations from several subjects and then calculating the correlation coefficient as if the data were a simple sample. This note is a response to several letters about the appropriate analysis for such data.

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    Repeated measurements of intramural pH and PaCO2 for eight subjects2

    The choice of analysis for the data in table I depends on the question we want to …

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