THE LAW OF INITIAL VALUES - A 4 FACTOR THEORY

The effects of four factors, measurement error, skewness, floor/ceiling effects, and reactivity, on the correlation between change and initial level were studied using computer simulations. A total of 24 conditions were simulated: the four factors being examined together with variations in the direction of change, level of measurement error and/or strength of effect. In each condition ten random samples of 50 cases were generated by SPSSX, and compute functions were used to model the effects of interest. Three factors were capable of producing a negative correlation (the Law of Initial Values, LIV): measurement error, skewness, and floor/ceiling effects. Two factors yielded a positive correlation (anti-LIV effect): reactivity and skewness. The issue of how to determine which factor is operating in a given situation is discussed.