Given a scientific passage and an answer, generate a question for the given answer.

Passage: The mechanical advantage of a machine is the factor by which the machine changes the input force. It equals the ratio of the output force to the input force. A wheel and axle may either increase or decrease the input force, depending on whether the input force is applied to the axle or the wheel. Answer: mechanical advantage
What is indicated by the ratio of the output force to the input force in a machine?