THE LAW OF INITIAL VALUES - 5 FACTORS OR 2

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 (r(dx)). 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 r(dx). When the variance increased, r(dx) became positive; when the variance decreased, r(dx) became negative. These five factors appear to affect r(dx) 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.