PROSPERO or INPLASY: Which Registry, and When

Prospective registration is now expected for systematic reviews, and most journals will ask for a registration number at submission. The two registries researchers actually choose between are PROSPERO and INPLASY. They differ in cost, speed, and scope in ways that matter to the decision.

The short answer

Register with PROSPERO if your review concerns a health-related outcome and you can accommodate the wait. It is free, it is the registry reviewers recognise by default, and some journals name it specifically.

Register with INPLASY if PROSPERO will not accept your review because it falls outside their health remit, or if a deadline makes their queue impractical.

Side by side

Where PROSPERO refuses registrations

This is the most common reason researchers end up at INPLASY, and it catches people late. PROSPERO requires a health-related outcome. Reviews that are declined on scope grounds regularly include:

• Educational interventions with attainment rather than health outcomes
• Reviews of methods, instruments, or measurement properties
• Purely economic or policy analyses with no health outcome
• Scoping reviews, which PROSPERO does not register at all

The last is worth emphasising because it surprises people. If you are running a scoping review, PROSPERO is not an option regardless of subject matter. Open Science Framework is the usual alternative there.

Timing, and the trap in it

Registration must be prospective. In practice that means before screening is complete, and certainly before data extraction begins.

PROSPERO's queue is the practical difficulty. Turnaround varies and has at times run to several weeks. Researchers working to a submission deadline sometimes begin screening while the registration sits in the queue. This is acceptable: what matters is the date you submitted the registration, not the date it was published, and the record shows both.

What is not acceptable is starting extraction, then registering, then presenting the review as prospectively registered. A reviewer comparing your registration date to your reported timeline will see it.

What to write in the registration

The registration is not a formality to be completed quickly. It is the document a reviewer will compare your manuscript against, and vagueness in it costs you later.

Be specific about:

• The PICO elements, including how you will define the population and which comparators are eligible.
• The primary outcome, singular. Registrations listing eight co-primary outcomes invite the objection that you selected the one that worked.
• The analysis plan, including the effect measure, the model, and how you will handle heterogeneity. Naming random effects here forestalls the common reviewer question about model choice.
• Prespecified subgroups. A subgroup analysis is credible in proportion to its being registered in advance. One registered subgroup analysis carries more weight than five discovered afterwards.

Choosing, in practice

For a clinical or public health review with a normal timeline, register with PROSPERO. The recognition is worth having and the cost is nothing.

For a review outside health, or a scoping review, PROSPERO is unavailable, and the choice is between INPLASY and OSF.

For a health review against a hard deadline, INPLASY's turnaround is a legitimate reason to choose it. Note the registry and number in the manuscript exactly as you would with PROSPERO, and expect no difficulty from the majority of journals.

Whichever you choose, register before you extract, be specific, and declare deviations. Those three things matter considerably more than which registry name appears in your methods section.