Another key point of Daniel’s experiment is that it was prospective: the groups were chosen in advance. By contrast, suppose that you see twenty people in an infomercial who all say they lost weight on a diet. That seems like a pretty large sample size, so some viewers might consider it convincing evidence. But that would amount to basing their decision on the experience of people who already had a good response. For all you know, for every person who lost weight, ten others just like him or her tried the diet and had no success. But of course, they weren’t chosen to appear on the infomercial.
