. An educational psychologist is interested in comparing problem solving skills of three different groups of adolescents: 1) those who haves not yet reached Piaget's formal operations stage; 2) those who have just reached the formal operations stage; and 3) those who have been in the formal operations stage for more than six month. The dependent variable is the number of problems solved correctly in six trials of a problem- solving task. To get a better picture of the relationship between these variables, the researcher decides to use a standardized measure of creativity as a covariate in the analysis. The data are: