Lesh, R., Post, T., & Behr, M. (1988). Proportional Reasoning. In J. Hiebert & M. Behr (Eds.) Number Concepts and Operations in the Middle Grades (pp. 93-118). Reston, VA: Lawrence Erlbaum & National Council of Teachers of Mathematics.
NOTE: the resource above is an excerpt from the report College and Career Readiness Standards for Adult Education (U.S. Department of Education, Office of
Vocational and Adult Education). Download the full report here.