The alternate-form reliability of a survey instrument, like a psychological test, helps to overcome the “practice effect”, which is typical of the test-retest reliability. The idea is to change the wording of the survey questions in a functionally equivalent form, or simply to change the order of the questions in the first and the second survey of the same respondents.
A common quantitative measure of the alternate-form reliability is the value of the correlation coefficient between the results obtained in two surveys – with the initial and re-worded questions.