This article presents the results of two experiments addressing the relation of reasoning skill to student grade, ability, and knowledge levels. In the first experiment, three levels of students within each of four grades-5, 7, 9, or 11-were designated as intellectually gifted, average, or below average. They were given three tasks involving everyday problems for which they provided solutions and justifications. The second experiment included the measurement of domain knowledge with grade and ability level. Measures of informal reasoning showed a substantial relation between ability level and performance, with knowledge significantly related to performance measures, such as number and type of reasons generated, but not to measures involving soundness or acceptability of arguments, which were explained by ability level. Grade was related only to an increase in personal and broadly defined social reasons; other effects were ''washed out'' by knowledge. The findings were interpreted in terms of a two-component model of informal reasoning, a knowledge-experiential component and an informal reasoning skill component based on the acquisition of argumentation-based language structures termed conventions of reasoning. Consideration of the relation of reasoning to learning and to instruction emphasized the importance of teaching informal reasoning skill.
