What is wrong? Explain what is wrong with each of the following randomization pr

Question

What is wrong? Explain what is wrong with each of the following randomization procedures and describe how you would do the randomization correctly. (a) Twenty students are to be used to evaluate a new treatment. Ten men are assigned to receive the treatment, and 10 women are assigned to be the controls. (b) Ten subjects are to be assigned to two treatments, 5 to each. For each subject, a coin is tossed. If the coin comes up heads, the subject is assigned to the first treatment: if the coin comes up tails, the subject is assigned to the second treatment. (c) An experiment will assign 40 rats to four different treatment conditions. The rats arrive from the supplier in batches of 10 and the treatment lasts two weeks. The first batch of 10 rats is randomly assigned to one of the four treatments, and data for these rats are collected. After a one-week break, another batch of 10 rats arrives and is assigned to one of the three remaining treatments. The process continues until the last batch of rats is given the treatment that has not been assigned to the three previous batches.

Explanation / Answer

3.34

Solution:

(a) There is a problem of confounding, since it will be inclear whether it was the treatment or the gender which influenced the outcome. To prevent this type of confounding when assigning the treatment it should be done randomly over both males and females.

(b) In theory this is a good idea, the only problem is that the number of heads on 10 tosses may not be exactly 5. Therefore there may not be an equal number in each treatment. To ensure that there is an equal number in each treatment the 10 names could be put in a hat and the first 5 names to be pulled out would be put one treatment and the other five names in another treatment.

(c) The 10 rats in a batch migt be similar to one another in some way. For example, they might be siblings, or they might have been exposed to unusual conditions during shipping. (The safest approach in this situation would be to treat each batch as a block, and randomly assign two or three rats from each batch to each treatment.)

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