What is a randomized clinical trial? Why would we want to randomize?

Case Study 5

VITAMIN E SELENIUM, AND PROSTATE CANCER: A RANDOMIZED CLINICAL TRIAL

A number of studies have been conducted over the years to determine whether vitamins or other micronutrients can decrease the risk of cancer, heart disease, and stroke. The results have not been encouraging. Klein and colleagues (2011) report the results of a randomized clinical trial (RCT) called the Selenium and Vitamin E Cancer Prevention Trial (SELECT). Men from 427 study sites were randomized between August 22, 2001, and June 24, 2004, to one of four groups:

1. Vitamin E only (n=8,737)

2. Selenium only (n=8,752)

3. Both Vitamin E and Selenium (n=8,702); or 

4. Placebo (n=8,696). 

To be eligible, subjects had to have a negative PSA test (< 4.0 ng/ml), a negative digital rectal exam, and be either 50 years and older (blacks), or 55 years and older (all other races). 

QUESTIONS

1. What is a randomized clinical trial? Why would we want to randomize? (150 words minimum)

2. The study used a randomized block design in which a “block” was one of the 427 study sites. Such a design ensures a balance if the four groups in each study site. Each site supplied an average of 83 men. Suppose Study Site A recruited 100 men.

a. how many men would be receiving vitamin E and selenium? 

3. Stratified randomization is similar, although it may involve more than one variable. Suppose the study was block randomized but also stratified by age (< 65 and > 65) and that you wanted 60 percent of men to be under 65. 

a. Of the 100 men in Study Site A, how many men under 65 would be in the placebo group? 

4. Describe the integrity of the randomization process and potential biases.  (150 words minimum)

5. Suppose this study were a cohort study, and subjects self-selected into the four groups. Assuming that the more health-conscious subjects would have a higher probability of choosing the vitamin E and/or selenium group, and that lower BMI and better dietary habits decrease the risks of prostate cancer, what would be the effects of this bias on the outcomes of the study?  

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