Preregistering a trial prevents outcome switching, selective reporting, and exploratory re-analysis presented as planned confirmation — by time-stamping the study's intentions before the data exist. Since 2000, U.S. law has required most clinical trials to register in the public ClinicalTrials.gov database run by the National Library of Medicine, including their planned primary outcomes, and readers can compare the registered plan with the published paper. That comparison, often taking minutes, is the whole enforcement mechanism.
Engevity News publishes information, not medical advice. This piece explains a research-integrity practice; decisions about joining or relying on a trial belong with clinicians.
What does preregistration actually record?
A registration entry files the study's skeleton in advance: the primary outcome — the single main measure the trial is designed to test — plus secondary outcomes, the planned sample size, the comparison groups, and key analysis choices. Registration does not peer-review the science or guarantee the trial is worthwhile. It commits the authors to a public definition of success before results arrive, which changes what they can credibly claim afterward. A trial that promised to measure heart attacks and reported cholesterol instead has answered a different question than the one it set out to ask — and registration makes that visible.
What is outcome switching?
Outcome switching is replacing a study's registered primary outcome with a different, more flattering one at publication. A trial measuring a primary outcome that came out null may report a secondary outcome that happened to look good, in the prominent position, without disclosure. In 2009, a study in JAMA led by An-Wen Chan compared registered protocols with subsequent publications and found that a substantial share of trials had discrepancies between registered and published outcomes, with unregistered outcomes added and unfavorable ones quietly dropped or reframed. Each switch is invisible in the final paper, which is exactly why the practice thrives where registration is not checked.
What else does a registered plan foreclose?
Several quieter flexibility moves, collectively sometimes called questionable research practices, are foreclosed or exposed by preregistration.
- Stopping rules: deciding to stop early once results look favorable, rather than at the planned sample size.
- Subgroup storytelling: finding one subgroup where the effect appears and presenting it as the finding.
- Moving the goalposts on statistics: switching tests or thresholds until significance appears.
- Selective publication: abandoning studies whose results disappoint, leaving the record biased toward success.
None of these requires dishonesty in the moment; each is a small drift that aggregated data and a public plan can catch.
How do we know preregistration matters?
The evidence comes from comparisons of registered plans against publications, and from trials registered before and after the requirement. Chan's 2009 JAMA analysis and subsequent work found outcome discrepancies in a meaningful fraction of trials, and studies comparing results by registration status have reported that adverse outcomes appear more often in prospectively registered trials than in those registered later or never — consistent with selective reporting filling the gap. Regulatory action followed the evidence: the 2000 requirement created ClinicalTrials.gov registration, and later rules under the Food and Drug Administration Amendments Act framework added results reporting obligations. The mechanism is simple: what is written down before cannot easily be unwritten after.
Does preregistration apply outside medicine?
Increasingly, yes. Psychology and economics adopted preregistration widely after their replication reviews in the 2010s, because flexible analyses were identified as a driver of irreproducible findings. The logic transfers wherever a hypothesis can be stated before data collection: the commitment costs nothing when the researcher intends a confirmatory test, and it only binds those who intended to keep their options open. Preregistration does not prevent fraud, and it does not make a study good — a poorly designed trial preregistered is still a poorly designed trial, now easier to see.
What are registered reports, and how do they go further?
Registration records the plan; a publishing format called registered reports changes the incentive itself. In this format, introduced at journals in the 2010s and now used across psychology, medicine, and beyond, researchers submit the study's design and analysis plan for peer review before data collection begins. If reviewers approve, the journal accepts the paper in principle — meaning the study will be published regardless of how the results turn out. The consequences are direct: null findings reach the literature instead of file drawers, outcome switching becomes contractually visible, and the reward shifts from a pretty result to a sound question. The format has costs — it slows planning, fits confirmatory rather than exploratory work, and covers only participating journals — but early analyses of published registered reports found far fewer positive-result distortions than the conventional literature. For readers, a published registered report carries a scarce property: the knowledge that the finding survived a design built before anyone, including its authors, knew the answer.
How can a reader use the registry?
The check is fast and public.
- Find the trial's registration number, usually printed in the paper's abstract or methods.
- Open the entry at ClinicalTrials.gov and read the registered primary outcome and planned sample size.
- Compare against the published paper's headline outcome.
- Note whether registration predates the trial's start — late registration is itself a signal.
A paper whose reported outcome matches a prospectively registered plan has passed a test most readers never think to run. A paper whose outcome does not match has not necessarily reported falsely — but it has reported something other than what it promised, and the reader now knows.
For more context, read How to spot a predatory journal.
For more context, read one study is not proof.
For more context, read How peer review works, and what it doesn't catch.