Source and Evidence Policy
A piece of content is only as trustworthy as the sources beneath it. This page explains which types of sources we prioritise, which we refuse to treat as evidence, and how we behave when the evidence conflicts.
Our order of priority
When preparing content, we approach sources in roughly this order:
- Official and international institutions: publications of bodies with public accountability, such as health authorities, space and science agencies, and national statistics or standards institutes.
- Peer-reviewed scientific literature: research that has passed independent expert review; accumulated evidence at the level of systematic reviews and meta-analyses rather than single studies.
- Competent professional bodies: guidelines and consensus statements from specialist associations.
- Quality secondary sources: well-organised accounts that rest on the layers above, such as textbooks and institutional educational material.
If a claim cannot be verified in any of these layers, we either leave it out or state its uncertainty plainly.
What we do not count as a source
Some kinds of text are not evidence for us, however professional they look. Product marketing copy is not a source: a manufacturer's claims about its own product cannot enter our content as fact unless independent evidence exists. The same rule applies to promotional "research" summaries, social media posts of unclear origin, anonymous claims, and accounts based purely on personal experience. Personal experience is not worthless; but on its own it cannot support generalisation, and it appears in our content only when labelled as experience.
How we show our sources
Our guide and lesson pages end with a source note summarising the types of sources the content rests on, naming institutions where appropriate. The aim is not an academic bibliography; it is that on every page, the reader can answer the question "where does this information come from?". In statements involving numbers and dates, we match the confidence of our wording to the strength of the evidence: where we are not certain, we say "approximately".
When the evidence conflicts
Not every question in science has a settled answer. Where studies contradict each other, we do three things: we state the uncertainty openly, we summarise both sides of the debate, and we base any practical suggestions on the most cautious interpretation. We do not overturn established knowledge on the strength of one new study; when the balance of evidence genuinely shifts, we do not hesitate to update the content. That work is governed by our Freshness and Maintenance Policy.
A working version
This policy is a working version. As we refine our criteria for evaluating sources, we will update this page. If you believe a statement on the site rests on weak grounds, please tell us through the process in our Corrections Policy; such reports improve the policy itself.