When there are several scientific studies on something and they are all opposing!

When there are several scientific studies on something and they are all opposing each other?

what are or who are you to believe?

There are an incredible number of variables in any given study that generally make them difficult to compare, once you dig into the details.

For example, you might read in the newspaper that a recent study indicates that St. John's Wort is NOT useful for depression, and then read a study that says it IS useful for depression. If you read the details, though, you'd find (in this recent example) that when used for minor depression, St. John's Wort can help tremendously, but when use for major depression, it rarely helps and can even cause some negative effects.

Minor depression and major depression are two very different things, but the newspaper headline writers (and even the reporters who write the body of the articles) rarely bother to include those important details, instead looking for the more sensational and interesting-seeming summary.

Even in cases where the study is reasonably-well reported, there are still rarely enough details to make useful comparisons. Were the people being tested old or young, did they eat well and take nutritional supplements or were they more average, did they participate in the test over a long period of time or was it just a weekend thing?

Sometimes the only way to really determine the outcome of a series of studies is to wait until a summary study comes out, that combines all of the information from a bunch of studies (commonly 20 or more) in order to come to a useful conclusion.