PC program for assessing the relationship between rate of change and initial value

A menu-driven PC program implementing Blomqvist's [ J. Am. Stat. Assn. 72, 746–749, 1977] method for assessing the relationship between rate of change and initial value is described, illustrated and made available. It is shown that the naive approach to this problem—computing the correlation between the initial value and either the amount or rate of change—results in a negatively biased estimator. The extent of this bias can be dramatic and may lead investigators to conclude that a negative correlation is present when none exists; or that there is no correlation when in fact the correlation is positive. Blomqvist's (maximum likelihood) estimator avoids this bias, and is obtained by a transformation of the naive estimator.