What is a scientific claim?
Not every sentence that begins "research shows that..." is scientific. What makes a claim scientific is not who says it or how technical it sounds, but how it can be tested.
Short answer
A scientific claim is a statement about facts, framed clearly enough to be tested by observation and measurement. What makes it scientific is not its subject matter but that it is testable, supported by evidence, and open to being abandoned if it turns out to be wrong.
Not every sentence is a claim, and not every claim is scientific
A rough sorting of sentences helps. Value judgements ("watching the sunrise is beautiful") rest on personal criteria and are not tested as true or false. Truths by definition ("a triangle has three sides") require no observation. Factual claims say something about the world: "the Moon is receding from the Earth by a few centimetres each year." Scientific claims belong to this third set, but not every sentence that looks factual meets a scientific standard. What decides the difference is whether the claim has been built in a way that lends itself to testing.
Four qualities of a scientific claim
Clear definition: the terms in the claim must be defined in measurable form; in the phrase "it raises your energy", what is energy to be measured with? Testability: predictions that can be compared with observation must follow from the claim; we treat this condition in detail in our guide to testability. Falsifiability: it must be possible to say in advance which observation would refute the claim. Repeatability: the result must not appear only in the hands of one person or one laboratory; independent teams following the method should reach a similar result.
Who carries the burden of proof?
The ground rule of scientific argument is this: the burden of proof lies with whoever makes the claim. Saying "you cannot prove that this effect does not exist" is not offering evidence; it is an attempt to hand the burden of proving a negative to the other side. A second rule goes hand in hand with it: extraordinary claims require extraordinary evidence. Modest evidence is enough for the claim that "this tea keeps you warm"; for the claim that "this tea renews cells" we are entitled to expect much stronger and independent evidence. These two rules are not tactics for winning arguments; they are filters that protect limited time and attention against claims that offer no evidence. When evidence arrives, the door is always open.
Evidence has grades too
Evidence in science is not all of one kind; there is a rough ordering of strength. Personal accounts and single experiences sit at the bottom; they may be sincere, but they are the kind of evidence most open to error. Observational studies show relationships yet struggle to prove a cause; we explain that limit in our guide to correlation and causation. Controlled and blinded trials are stronger, and at the top sits the joint assessment of many mutually independent studies. "According to one study" and "according to the shared result of dozens of independent studies" do not carry the same weight.
The distance between the headline and the finding
We meet most scientific claims through the language of news, and a great deal is lost in that translation. A researcher's sentence, "we observed an association in a limited sample", can travel into a headline as "scientists have proved"; a preliminary study in mice can turn into good news generalised to humans. The problem is usually not bad faith but compression: margins of uncertainty, sample size and warnings that "more research is needed" find no room in a headline. A few practical filters are available to the reader. Does the report say who the original study was carried out with, and how many people took part? Are definite verbs like "proved" used, or cautious phrases such as "an association was found"? Does the finding rest on a single study? The answers to these questions usually show, on their own, the distance between the excitement of the headline and the weight of the evidence.
What looks scientific but is not
Some claims use the vocabulary of science without using its method; this is called pseudoscience. It has familiar markers: unmeasurable concepts ("vibration", "blockage"), flexible interpretations that can explain any outcome, emphasis on authority and titles in place of evidence, declaring a conspiracy rather than answering criticism, and never correcting itself however many years pass. No single marker settles the matter; but if several appear together, caution towards the claim is warranted. These criteria are the most practical tool set a consumer of information can have, and you can find the standard of evidence we observe in the content on this site on the Source and Evidence Policy page.
Frequently asked questions
Is a scientific claim always true?
No. Being scientific is not a guarantee of truth but a property of testability. A scientific claim can turn out to be false under testing; the strength of science lies in its ability to weed out errors.
Does expert opinion count as evidence?
Expertise is valuable for reading evidence correctly, but on its own it is not evidence. An expert's view is also judged by the studies it rests on; a title does not stand in for a claim.
Why isn't my personal experience enough as evidence?
Experience is real but it is a single case; it cannot be separated from factors such as the placebo effect, chance and selective recall. This is why science treats experience as a starting point and proceeds by controlled testing.
Is everything that cannot be tested worthless?
No. Art, philosophy and faith are other ways of making sense of the world. The criterion of scientificity applies only to statements that claim certain knowledge about facts.
This guide draws on the concepts and criteria set out in standard textbooks in the philosophy of science and in research methods.