Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: The population growth rate is how quickly a population changes in size over time. The rate of growth of a population may be positive or negative. A positive growth rate means that the population is increasing in size because more people are being added than lost. A negative growth rate means that the population is decreasing in size because more people are being lost than added. Answer: population growth rate
Output:
How quickly a population changes in size over time is known as what?