Derived variables are survey variables whose values are determined by calculations done on responses to questions and other derived variables. They are usually used at the analysis phase but they also have sophisticated uses at the interviewing phase.
A survey includes a question on a respondent's age, and another on their gender. A derived variable could be composed to show analyses of compound groups such as "Male, age group 1", "Female, age group 1", "Male, age group 2", and so on.
There are many other uses for derived variables and Snap Survey Software enables any number to be created for a survey.