Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another. Answer: variable
In a controlled scientific study, what do you call a factor that can take on different values?