Question 3.57

3.57 What is wrong?

Explain what is wrong with each of the following randomization procedures, and describe how you would do the randomization correctly.

  1. A list of 50 subjects is entered into a computer file and then sorted by last name. The subjects are assigned to five treatments by taking the first 10 subjects for Treatment 1, the next 10 subjects for Treatment 2, and so forth.

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  2. Eight subjects are to be assigned to two treatments, four 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.
  3. An experiment will assign 80 rats to four different treatment conditions. The rats arrive from the supplier in batches of 20, and the treatment lasts two weeks. The first batch of 20 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 20 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.

3.57

(a) Not all subjects have an equal chance to be in each group (alphabetical is not random). (b) More than 4 subjects (even all 8) could end up in the same group. (c) Batches of rats could be different, so randomization should be done so that each rat is treated separately or batches of rats are divided randomly among treatments equally.