A New Method for Investigating the Relation between Change and Initial Value in Longitudinal Blood Pressure Data: II. Comparison with Other Methods

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Abstract
Whether initial value affects blood pressure change or not is of interest in longitudinal blood pressure studies. The change/initial value relationship is. however, biased by the regression towards the mean phenomenon, as blood pressure measurements contain random errors (short-term intn-individual variation and measurement errors). Two methods (here called C and D) of avoiding this bias have earlier been proposed and used in population studies in South Wales and Framingham, with conflicting results. In the preceding paper a new method of avoiding the bias has been presented. In the present paper a comparison of the new method with the two earlier is made by applying them to the same data set. Method C indicates, as in South Wales, a highly significant positive relationship, while method D (as in Framingham) and the new method give a regression coefficient close to zero. The structure of the three methods is analysed. Method C is shown to be valid only under very restricted conditions and is best avoided. Method D gives a fairly accurate result, but underestimates the coefficient by approximately 30% in this case. The new method proposed seems to be the best method hitherto for this kind of analysis.

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