This study attempted to search for a learning hierarchy among the skills comprising formal operations and the integrated science processes. Data were obtained from two instruments administered to 741 high school science students. The Group Assessment of Logical Thinking (GALT) measured performance on six Piagetian cognitive modes: controlling variables and proportional, probabilistic, correlational, conservation, and combinatorial reasoning. The Test of Integrated Process Skills (TIPS II) measured performance on five integrated process skills: identifying variables; hypothesizing; operationally defining; designing experiments; and graphing and interpreting data. Data were analyzed by computer programs based on the theoretic method and a probabilistic method (which evaluates the degree to which an "a priori" hierarchy provides a reasonable model for the data). The latter method also permits a statistical comparison among two or more alternate hierarchies which are fitted to the same data sample. Results demonstrated the existence of linear and branching relationships among the skills examined, most of which fit the logical hierarchies which guided curriculum development and classroom practices in the past. Results also indicate that students may not be able to acquire certain scientific process abilities until prerequisite cognitive skills are developed. (JN)