One hundred and sixty people who suffer from painful diabetic neuropathy have volunteered to participate in a study. Eighty are selected at random and are given the drug gabapentin, which (although originally intended to prevent epileptic seizures) has properties that may make it useful to alleviate neuropathy. The remaining participants are given a placebo. A neurologist evaluates the symptoms of all volunteers after two months to determine if there has been substantial improvement in the severity of the symptoms. Suppose the volunteers were first divided into men and women, and then half of the men were randomly assigned to the new drug and half of the women were assigned to the new drug. The remaining volunteers received the placebo. This would be an example of