A prominent surgeon came to give a special lecture at the School of Medicine. He expounded about the great advance that he had made in a specific surgical procedure.
At the end of the lecture he drew thunderous applause from the audience. At first it seemed like there would be no questions, but then a young student in the front row raised her hand. "Did you use any controls?" she asked.
The surgeon seemed to be offended by this question. "Controls?" he asked. "Are you suggesting that I should have denied my surgical advance to half of my patients?"
The rest of the audience grew very quiet. But the young woman was not intimidated. "Yes," she said, "that's exactly what I meant."
The surgeon grew even angrier at this, slammed his fist on the podium and shouted "Why that would have condemned half of my patients to certain death!"
There was silence for a few seconds. Then the entire auditorium burst out in laughter when the young woman asked "Which half?"