How did the researcher collect sample data in each group that is representative of the population?

Scientists at Wake Forest Baptist Medical Center have found new evidence that mindfulness meditation reduces pain more effectively than placebo. The research, published in the Nov.11, 2015 issue of the Journal of Neuroscience, showed that study participants who practiced mindfulness meditation reported greater pain relief than placebo. Significantly, brain scans showed that mindfulness meditation produced very different patterns of activity than those produced by placebo to reduce pain. The study used a two-pronged approach -- pain ratings and brain imaging -- to determine whether mindfulness meditation is merely a placebo effect. 75 healthy, pain-free participants were randomly assigned to one of four groups: mindfulness meditation, placebo meditation ("sham" meditation), placebo analgesic cream (petroleum jelly) or control. Pain was induced by using a thermal probe to heat a small area of the participants' skin to 49 degrees Centigrade (120.2 degrees Fahrenheit), a level of heat most people find very painful. Study participants then rated pain intensity (physical sensation). The mindfulness meditation group reported that pain intensity was reduced by 27 percent. In contrast, the placebo cream reduced the sensation of pain by 11 percent. Mindfulness meditation also was significantly better at reducing pain intensity the placebo meditation. The placebo- meditation group had relatively small decreases in pain intensity (9 percent). The study findings suggest that placebo meditation may have reduced pain through a relaxation effect that was associated with slower breathing. "Based on our findings, we believe that as little as four 20-minute daily sessions of mindfulness meditation could enhance pain treatment in a clinical setting. However, given that the present study examined healthy, pain-free volunteers, we cannot generalize our findings to chronic pain patients at this time." Question: How did the researcher collect sample data in each group that is representative of the population