$5.$ A clinical trial was conducted to determine whether a hormone treatment benefits women who were treated previously for breast cancer. Each subject entered the clinical trial when she had a recurrence. She was then treated by irradiation and assigned to either a hormone therapy group or a control group. The observation of interest is the time until a second recurrence, which may be assumed to follow an exponential distribution with parameter $($hormone therapy group$)$ or $($control group$).$ Many of the women TABLE $7\mathrm{.}2$ Breast cancer data. did not have a second recurrence before the clinical trial was concluded, so that their recurrence times are censored. In Table $,$ a censoring time means that a woman was observed for months and did not have a recurrence during that time period, so that her recurrence time is known to exceed months. For example, $15$ women who received the hormone treatment suffered recurrences, and the total of their recurrence times is $280$ months.