What a randomised trial does not need to record

Topic: Studies, data, and evidence · more questions on this topic
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A trial of a new blood-pressure drug against an existing one will recruit at least 1000 adults aged 30 to 60 with systolic pressure at least 140 mm Hg, of any sex, fitness, occupation, or diet. The only columns required for the comparison are the assigned drug and the blood pressure before and after treatment. Why can age, sex, fitness, occupation, and diet be left unrecorded?

Worked answer (click to show)

Because the drug is assigned at random, the two treatment groups have, apart from chance, the same distribution of age, sex, fitness, occupation and diet, and of every other variable, measured or not. None of them can then explain a difference in blood pressure between the groups: any systematic difference must be due to the drug. With 1000 or more subjects, chance imbalances are small. The other variables would still be useful for finding out whether the drug works better for some people than others, or for making the comparison more precise, but they are not needed for a valid comparison.