EXAMPLE 3 Treating cocaine addiction

Cocaine addicts need the drug to feel pleasure. Perhaps giving them a medication that fights depression will help them resist cocaine. A three-year study compared an antidepressant called desipramine, lithium (a standard treatment for cocaine addiction), and a placebo. The subjects were 72 chronic users of cocaine who wanted to break their drug habit. An equal number of the subjects were randomly assigned to each treatment. Here are the counts and percentages of the subjects who succeeded in not using cocaine during the study:

The sample proportions of subjects who did not use cocaine are quite different. In particular, the percentage of subjects in the desipramine group who did not use cocaine was much higher than for the lithium or placebo group. The bar graph in Figure 24.1 compares the results visually. Are these data good evidence that there is a relationship between treatment and outcome in the population of all cocaine addicts?