In a study designed to measure whether the number of hours of sleep significantly predicts higher test grades, researchers vary the number of hours of sleep that subjects get per night. this method of manipulating one variable to measure change in a second variable is called:

experimental method.

Explanation:

Based on the information provided within the question it can be said that the method that is being described is called the experimental method. This method, like described in the question, refers to manipulating one variable in order to see the effects that it causes in another variable, and is done in order to test the hypothesis and decide whether it s refuted or not.

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