 A famous controversy in which the results from observational studies and those from RCTs conflicted was that over the benefits and safety of hormone replacement therapy (HRT) in the early 2000s (Vandenbroucke 2009). HRT seemed protective for coronary heart disease in observational studies, whereas RCTs indicated an increase in the first years of use. For breast cancer, combined hormone preparations showed a smaller risk in an RCT than in observational studies. In the end it turned out that the timescale of the effects was responsible, and that because of the way they are typically run, observational studies got some issues right and RCTs others: