Completion codes, without the support tickets.

A panel study is a loop with three URLs. First, the study URL: the link you paste into Prolific, pointing at your survey, with placeholders that Prolific fills in per participant:

When participant 5f8a… clicks it, Prolific replaces the {{%…%}} placeholders with their real IDs. Your survey tool must capture those parameters and store them with the response — that is how you later match a row of data to a submission you are approving.

Second, the completion URL: where you send participants when they finish. Prolific gives you this on the study page; it looks like https://app.prolific.com/submissions/complete?cc=A1B2C3D4. Redirecting there marks the submission complete automatically.

Third, the screen-out route — same idea, different code — covered in section 03.

The automatic redirect is the good experience: the participant clicks Submit, sees your thank-you screen for a beat, and lands back on Prolific with everything recorded. But redirects live in the real world: a participant closes the tab early, a browser extension eats the navigation, a phone locks at the wrong moment. The fix costs nothing — also print the code on the thank-you screen, so anyone who missed the redirect can paste it into Prolific manually.

The failure mode to avoid at all costs is the silent dead end: a thank-you page with no code and no redirect. The participant has no way to prove completion, Prolific shows the submission as awaiting, and your inbox fills up. Every “survey tool ate my completion code” thread you have ever seen is this.

In Plumeform both halves are one settings panel: set the completion code and paste the redirect URL, and the thank-you screen shows the code and forwards automatically. The share panel's launch kit generates the study URL with the placeholders already wired.

If your study has eligibility questions, some participants will be disqualified after starting. They still need to leave through a door, not a wall. Prolific studies can define multiple completion codes with different outcomes — one for genuine completions, one for screen-outs — so a disqualified participant returns with the screen-out code and the submission resolves correctly instead of hanging.

Your survey tool therefore needs two endings: the thank-you screen (completion code + redirect) and a separate screen-out screen with its own code and its own redirect. The screen-out copy matters more than people think — “you didn't qualify” reads as an accusation; “this study needs a specific group and this one isn't a match — your time is still appreciated” reads as respect. You will meet these participants again in your next sample.

Put the screener in the first minute. The earlier someone screens out, the cheaper it is for you and the fairer it feels to them.

Never launch a panel study without one dry run of the full loop. Open your own study URL with fake parameters — say ?PROLIFIC_PID=TEST123— and complete the survey. Check three things: the response stored your test ID, the thank-you screen shows the right code, and the redirect lands on Prolific's completion page (it will refuse your fake session, which is fine — you only care that you arrived). Then check the screen-out path the same way by answering as an ineligible participant.

The catch: that dry run is now a row in your data. Either delete it by hand before launch, or use a tool with a real test mode — Plumeform treats any link opened with ?test=1 as a dry run: flagged, excluded from results and quotas, and deletable in one click. Your rehearsals never touch your dataset.