Evidence should be put in context, or qualified in Wikipedia articles, especially for sources that should not be taken at face value. Some articles are tagged with {{qualify evidence}}, and this page helps describe what to do for them, and how to treat factual claims in general.
Things to check for scientific studies mentioned in articles:
In general:
Editors may need to:
Wikipedia generally avoids trying to predict the future, but in some cases does discuss things that are have not actually happened. In medical levels of evidence, expert opinion is considered the least reliable, when compared with conclusions supported by epidemiology or experiment. In general, including fields like business and government, expert opinions are wrong more often than not. (See e.g. Wrong by David H. Freedman.)
When reporting expert opinions, Wikipedia editors should seek out multiple independent perspectives. When there is only one source, that should be carefully reported as unconfirmed and the source should be characterized. For example, is a report on Internet traffic patterns and likely future bottlenecks something published by a business which has a stake in building more telecommunications services, or was it an impartial government report? Is a prediction about the number of deaths that would be caused by a certain type of earthquake made by a group seeking donations for preparedness, a government agency prioritizing resources, or a business that earthquake-proofs buildings?
Correlation does not imply causation. A study that shows the presence of peanut butter correlates with the presence of jelly does not imply that smearing peanut butter on a slice of bread will cause jelly to appear.
When describing studies that prove or suggest correlations, Wikipedia should make clear that this does not prove causation (unless there are other studies or sources which support causation).
Wikipedia should make a special effort to avoid "single study syndrome"[1][2] - reporting the results of a new study at face value as if it were a reliable, true, and complete picture of the subject. Keep in mind that:
Systems like living bodies, the human mind, and societies are extremely complicated, and a single study might not take all of that complexity into account (often because the true nature of the system has yet to be discovered). For example, a sociology study conducted at noon might conclude that Californians frequently travel outside their homes, whereas one conducted at 3am might conclude they almost never leave home. Over time, secondary social science sources should become aware of these conflicting studies; if that's the state of knowledge at the moment, Wikipedia should report the conflict rather than decide the issue one way or another. With even more time and more studies, sociologists might resolve the conflict by deducing that Californians tend to leave home during the day and stay home at night because they are asleep. When a new consensus emerges in secondary and tertiary sources, Wikipedia should report that. It should also beware of primary sources that challenge the consensus - expert evaluation is needed as to whether they clearly indicate a new conflict vs. spurious or mistaken results.
The scientific method demands reproducibility of results before concluding that a given claim is proven to a high degree of reliability. When citing primary sources like studies, Wikipedia should make clear where a given claim is in the social process of gathering evidence for and against. Science is a social enterprise; the elementary school "observation, hypothesis, experiment, conclusion" is only the beginning of the real process!
Wikipedia:Identifying reliable sources (medicine) has the full details, but essentially:
Remember that even clinical trials in humans have phases. Only about 18% of new drug candidates pass Phase II and only about 50% of those that make it to Phase III are approved.
Link to the article levels of evidence so readers can understand what it means that a study hasn't been reproduced, or that it used a randomized controlled trial vs. epidemiological study vs. animal experiment. (Good example: Postperfusion_syndrome#Evidence)
Where medical claims are unsupported or where the cited source is primary (thus lacking context) or not authoritative on medical matters (e.g. popular press):
Examples of dealing with contradictory results or shaky evidence:
The Reproducibility Project could reproduce only 36% of the published, peer-reviewed psychology experiments it tested.
Examples on specific topics: