Observational studies and randomized, controlled trials were compared to see if they find different treatment effects. Researchers looked at reports from 1985 to 1998 on various treatments like calcium-channel-blocker therapy and appendectomy. They found that in most cases, the treatment effects seen in observational studies were similar to those in randomized, controlled trials. Only in 2 out of 19 cases did the effects differ. Overall, there is little evidence that treatment effects in observational studies are consistently larger or different from those in randomized, controlled trials.