A preprint is a research paper posted publicly before peer review. medRxiv and bioRxiv, the two servers this site reads, screen for obvious problems — plagiarism, patient identifiers, dangerous claims — and post within days. Nobody has checked the methods. That is not a reason to ignore preprints, which is where much of the best work now appears first, but it is a reason to read them differently.
What the label means on this site. Items marked preprint are drawn from the servers and carry that source type until the paper is published, at which point the item is updated to say so and to note whether the findings changed. The preprints page keeps the tally: how many were published, how many changed, how many were withdrawn. In practice most are published, most change little, and a few change a great deal.
What to check, in order.
The design. Is it a registry cohort, a trial, a cross-sectional survey, a model? The design sets a ceiling on what the paper can claim, and no amount of sample size lifts it. A large cross-sectional study cannot show that one thing caused another.
The population and the numbers. Who was studied, how many, over what period, and whether the headline number is the main analysis or a subgroup. A finding in a subgroup of a subgroup is a hypothesis, not a result, however it is worded.
Does the abstract match the tables? The abstract is written last and written to be read. Where a result in the tables is weaker, non-significant or absent from the abstract, the tables are the paper.
Preregistration and data. Was the analysis plan filed before the data were seen? Are the data and code available? Neither guarantees anything, but both make it harder for a result to be the product of many tries.
Who paid, and who wrote. Funding is declared; read the declaration. Author affiliations tell you whether the people who made the data are the people analysing it.
Plausibility. Does the size of the effect fit what is known? A new drug that halves all-cause mortality in an observational study is more likely to be confounded than miraculous.
What peer review would and would not have caught. Reviewers check reasoning, methods and interpretation, and sometimes catch errors; they rarely re-run analyses, seldom see raw data, and never remove the incentive to make a result sound bigger than it is. A published paper is a reviewed preprint, not a different kind of object.
When to wait. A preprint that would change a driver's status waits for publication or replication before it does so here; it is filed, labelled, and given the new verdict rather than confirms or contradicts. The site reports what it says; the status of the question moves when the evidence has been through more hands.