proxy variable
a variable, b, used in place of another, a, when b and a are substantially correlated but scores are available only on variable b, often because of the difficulty or costs involved in collecting data for variable a. In other words, b is a substituted measure of a. For example, rather than reporting the precise income levels of their parents or guardians, survey respondents instead may be asked to indicate the highest level of education of each parent or guardian; in this case, education is a proxy for income. Also called surrogate variable.