Question: Don't do drugs! Cocaine addicts need cocaine to feel any pleasure, so perhaps giving them an antidepressant drug will help. A three-year study with 72 chronic cocaine users compared an antidepressant drug called desipramine with lithium (a standard drug to treat cocaine addiction) and a placebo. One-third of the subjects were randomly assigned to receive each treatment. Here are the results:

(a) Compare the distributions of relapse status for the three treatments.

(b) Are the differences among the three groups statistically significant? Give appropriate evidence to support your answer.