A new cream was developed to reduce the irritation caused by poison ivy. To test the effectiveness, researchers placed an ad online asking for volunteers to participate in the study, and 100 subjects replied. They are informed that one group will receive a new cream and the other group will receive a cream with no active ingredient. The researchers believe that sun exposure may affect the cream’s effectiveness. The subjects are then asked about their outdoor activities, and 42 of the subjects state they spend more than 50% of their day outside. All subjects are exposed to poison ivy and given their cream. They are asked to return in three days and report their level of irritation. Which of the following describes a randomized block design?

A. The subjects are grouped based on their outdoor activity. The 42 subjects are randomly assigned to the two treatment groups. The remaining 58 subjects are also randomly assigned to the two treatment groups.

B. The subjects are numbered 1–100, and these numbers are entered into a random number generator. Fifty numbers, ignoring repeats, are generated and these subjects receive the new cream. The remaining 50 receive the inactive cream.

C. The subjects are paired based on similar age. Two marbles, one red and one blue, are placed in a bag and mixed. If the first subject picks a red marble, then that subject will receive the new cream and the other subject will receive the inactive cream. If it is blue, then the opposite will occur. This is done for all 50 pairs.

D. The subjects are grouped based on their outdoor activity. The researchers flip a coin and if it lands on heads, then the group with the most sun exposure will receive the new cream and the other group will receive the inactive cream. If the coin lands on tails, then the group with the most sun exposure will receive the inactive cream and the other group will receive the new cream.