A study in Finland in 1998 suggested that vitamin E pills reduced the risk of prostate cancer in a group of male smokers. This was only one effect in a series of reported successes for vitamin E and other health conditions. A more recent study reported that selenium or vitamin E, used alone or in combination, did not prevent prostate cancer in a population of relatively healthy men ( JAMA , April 2009). Discuss the factors that can cause different medical studies to come to different conclusions.