This chart attempts to give a brief overview of what to take into account when judging what weight to give a source within medicine and health. It lists three important points that should always be taken into account: relevance, quality, and age — of which quality is the most important. The fourth point: whether a source is the best possible summarizes the previous, but may also require adding a few other parameters into the mix. How strictly to adhere to each point depends on what you are writing about, and how much research exists within the field.

• A relevant source is one where the topic is the same as for the statement you use it for. E.g. — if you're writing about methotrexate treatment for rheumatoid arthritis: the best possible source in one covering treatment options in general, or even specifically methotrexate. The second best is a source that is about a different treatment, but states your topic in passing: e.g. "DMARDs have benefits over methotrexate, because they do not cause leukopenia". Even less relevant is a paper or a related disease: e.g. on psoriatic arthritis. Entirely unrelated or marginally related sources should not be used.
• Quality assessment is performed according to the section on assessing evidence quality, but in essence means that higher quality sources trump lower quality sources — where the lowest quality sources are entirely avoided.
• Medical research moves in fits and starts: sometimes the state of published evidence can change dramatically from one day to the next, at other times progress takes many years. When comparing review articles and if all else is the same — more recent articles should be chosen. Major organizations often recommend relying on reviews of the evidence no older than 2 years for "best evidence" (providing they haven't been trumped by newer authoritative evidence). Rare diseases or areas where research moves slower are exceptions where Wikipedia takes a more pragmatic approach. In some fields such as anatomy (excluding microanatomy) it may be acceptable to use sources dating back 30+ years. They are however always trumped by newer sources of similar quality.
• Always seek the best source possible. It is unreasonable to expect editors to always use the best sources, but they should always try. Issues finding or using the best sources may be: pay-walls (see the Resource Exchange for help on accessing pay-walled content); difficulties in determining which source is the best; or it may just be unreasonably time-consuming to go through all possible sources.
  When you have several contenders for best possible source, and they don't agree on something make sure to give both viewpoints, but remember to avoid giving false balance.

Try to avoid bias when selecting sources. Bias can arise from your search criteria and search tools, and in which sources you have access to (see for example FUTON bias).^[1]