This study of Piagetian formal reasoning in seventh grade students reports the relationships between four aspects of the ability to control variables in an experiment and the relationships between those four aspects and other constructs. The four aspects of the ability to control variables identified are: (1) set up a controlled experiment, (2) assess an experiment done by another, (3) ignore the results of an uncontrolled experiment and recognize that it is uncontrolled, and (4) indicate what variables will affect the outcome of an experiment. The 124 subjects dealt with each of those questions on three different tasks. Measures of achievement (crystallized ability), field dependency (FDI), locus of control, divergent thinking, concept identification, and the Category Width Test were also employed. Results indicate that all true variance in the measures of formal thought is explained by these ability and personality measures. The interrelations among the factors studied are discussed, and brief comments about educational implications are made. Three tables present study results. (Author/BH)