This paper presents a review of the research on proportional reasoning. Methodologies used in proportional reasoning studies are presented first. The discussion is then organized around the following topics: strategies used to solve proportion problems, including erroneous strategies; factors that influence performance on proportion problems, both task-related and subject-related; training studies. The discussion is accompanied by suggestions for educational and research applications.For a mathematician, a proportion is a statement of equality of two ratios, i.e., a/b = c/d. Although most people are probably unaware of the mathematical definition of proportions, they do use them in familiar situations. Proportions are also widely used in more scientific contexts. Despite its importance in everyday situations, in the sciences, and in the educational system, the concept of proportions is difficult. It is acquired late (see, for example, Newton et al. 1981, or Pallrand, 1979. Moreover, many adults do not exhibit mastery of the concept (e.g., Capon and Kuhn, 1979).Because it is both useful and difficult to master, proportional reasoning has been the object of many research studies in the last 25 years. During this period, the research has become increasingly sophisticated, changing from a view of proportional reasoning as a global ability, or a manifestation of a general cognitive structure, to a more differentiated view focusing on describing the procedures used in proportional reasoning and how they are influenced by task and person parameters. However, as the studies became more differentiated in style some parameters were studied in detail without attention being paid to other tasks parameters. As a result, the body of research suffers from many gaps, it lacks cohesiveness and it is difficult to apply to mathematics education.It is the purpose of this paper to: (1) organize the research findings around the relevant task and subject parameters, (2) suggest research to fill the gaps in the literature, and (3)suggest ways to apply these findings in education. It begins with a discussion of the methodologies used in studying proportional reasoning.
M E T H O D O L O G YStudies of proportional reasoning have employed different methodologies and tasks. A list of the tasks used in various experimental studies can be found in