The law of initial values: five factors or two?
We (Jamieson and Howk (1992) Int. J. Psychophysiol. 12, 53-61) recently used computer simulations to study factors affecting the correlation between change and initial level (rdx). Four factors were identified: (1), measurement error; (2), floor/ceiling effects (the 'real Law of initial values'); (3), reactivity and (4), skewness. The present study used the same approach to show that a fifth factor, change in variance from pre to post, was also capable of influencing rdx. When the variance increased, rdx became positive; when the variance decreased, rdx became negative. These five factors appear to affect rdx through only two underlying mechanisms: regression to the mean and variance change. The difficulty of separating a 'real Law of initial values' from the other factors is discussed.
Jamieson
J
J
Psychology Department, Lakehead University, Thunder Bay, Ontario, Canada.
D000704:Analysis of Variance; D003198:Computer Simulation; D006801:Humans; D011603:Psychophysiology / Q000706:statistics & numerical data*; D012016:Reference Values