Which of the following could be an ideal randomized controlled experiment to study the desired causal effect?

A. Allow all students an extra day to study for the microeconomics exam. Then measure the exam score difference between the student with the highest score and lowest score respectively.

B. Allow the fifteen students with the highest grades in the class an extra day to study for the microeconomics exam. Then measure the exam score differences between students who got the extra day to study and those that did not.

C. Allow fifteen students, chosen at random, an extra day to study for the microeconomics exam. Then measure the exam score differences between students who got the extra day to study and those that did not.

D. All of the above could be ideal randomized controlled experiments.

The answer is: C) Allow fifteen students, chosen a random, an extra day to study for the microeconomics exam. Then measure the exam score differences between students who got the extra day to study and those that did not.

Explanation:

The whole idea behind a randomized controlled experiment is to reduce the sources of bias when testing the effectiveness of the hypothesis.

In this case the hypothesis probably was that the students that had an extra day to study will get a higher score in the test. In order to prove that hypothesis the students allowed an extra should be chosen at random. If you only choose the smartest students, then they will probably get higher scores than the rest because they usually do since they're smarter. Also if you give all the students an extra day, you wouldn't have any control group to compare.