For a group of long-distance runners who have been assigned to a program, a researcher wants to study whether there has been any significant improvement in endurance. The program uses creative...

For a group of long-distance runners who have been assigned to a program, a researcher wants to study whether there has been any significant improvement in endurance. The program uses creative coaching methods simulating real-life situations to increase the endurance ability of the athletes. The researcher plans to use the regression discontinuity design as the samples already fulfill the criteria of being long-distance runners. Which among the following criteria violates the ­requirement of the design?

a.            Assignment to the comparison group is based on the distance run.

b.            The cut-off score should not be close to the longest distance.

c.             The sample must be a mix of long- and short-distance runners.

d.            The experimenter must have control over the assignment.