Recruit participants through Prolific

On Prolific: the study link and the completion codes

In your Prolific study, select "I'll use URL parameters". Paste the participant link of Carrier with the Prolific placeholders: https://<your Carrier address>/experiment/<study id>?PROLIFIC_PID={{%PROLIFIC_PID%}}&STUDY_ID={{%STUDY_ID%}}&SESSION_ID={{%SESSION_ID%}}

Copy the first part with Copy participant link in the ⋯ menu of the builder.

Carrier records the three values. PROLIFIC_PID is the participant ID. STUDY_ID and SESSION_ID go in the studyId and sessionId columns of the export.

Then, under completion codes, make one code for each exit from your study. Select what Prolific does with each code. For example:

• C7XK2Q9P "completed": approve and pay.
• NM4TQ8ZL "no partner found": pay.
• AF2RW6JD "failed attention check": return or reject, as your ethics approval permits.
• SC9VH3NE "screened out": pay a screening fee.

Each code has a link, for example https://app.prolific.com/submissions/complete?cc=C7XK2Q9P. In the next steps, you paste these links into Carrier.

Step 1 — Read the participant id from Prolific

Click the study name at the top of the builder to open Experiment Settings. Under Completion Redirect, set Participant ID URL Parameter to PROLIFIC_PID.

Carrier then records each participant under their Prolific ID. Carrier also adds the ID when it sends them back to Prolific. For other platforms, use Custom (for example workerId), or keep pid.

Step 2 — Send finishers back with the "completed" code

In Redirect URL, paste the link of your "completed" code: https://app.prolific.com/submissions/complete?cc=C7XK2Q9P.

At the end of the study, the participant sees a "Study Complete" notice for three seconds. Then Carrier sends them to the link and adds &PROLIFIC_PID=<their id>.

If Redirect URL is empty, the participant sees a Study Complete page. Nothing sends them back.

Step 3 — Send participants with no partner back with their own code

In a study with group chambers, a participant can wait and never get a match.

In Not-Matched Redirect URL, paste the link of your "no partner found" code (…?cc=NM4TQ8ZL). If it is empty, Carrier sends these participants to the "completed" link.

Then, in the settings of each group chamber:

• Set Matching Timeout (seconds). This is how long a participant waits.
• Set On Matching Timeout to "End session". This setting sends the participant to the link.

Make the Prolific time limit much longer than your study plus this wait.

Step 4 — Send participants who fail the attention check back with their own code

Open the attention-check segment. In On Fail, Terminate session ends the study for a participant who fails.

Its Redirect URL is the destination of these participants. Paste the link of your "failed attention check" code (…?cc=AF2RW6JD). Each attention check has its own link. Thus different checks can use different codes.

Step 5 — Screen out ineligible participants with their own code

Open the survey segment with the eligibility question. Under Completion Conditions, add a rule:

• If field: the name of the question (here seen_before).
• is: = Yes.
• Redirect URL: the link of your "screened out" code (…?cc=SC9VH3NE).

Keep Record as on Screened out, the default for a new rule. The export then shows these runs as Screened Out, not as completed. The other choices are:

• Completed early, for a rule that ends the study correctly.
• Failed an attention check.
• Declined consent.

A participant who answers Yes leaves the study at that point. Add one rule for each answer that screens participants out. When all the links are in, click Deploy.

What participants see — finishing the study

Participants open the Prolific link, which has their ID. At the end, they see "Study Complete" for three seconds. Then Carrier sends them to the "completed" link with their PROLIFIC_PID.

In these clips, a substitute page shows the address that Prolific receives.

What participants see — screened out

An eligibility answer that matches the rule ends the study immediately.

Note: each redirect shows the same notice for all codes: "Study Complete" and "Thank you for participating. Your responses have been recorded." The message of the rule shows only when the rule has no redirect URL.

What participants see — failing the attention check

An incorrect answer to the attention check ends the study and sends the participant to its link.

If a debrief page is on, the participant first sees "Study Ended" and the debrief. Carrier sends them on only when they click through the debrief.

What participants see — no partner found

A participant who waits longer than the matching timeout goes to the not-matched link. In this demo the timeout is 15 seconds. In a real study it is some minutes.

Exits that do not go back to Prolific

Some exits do not redirect. These participants get back to Prolific only if they return the study themselves. The exits are:

• A session that you end from the dashboard.
• A participant who disconnects for longer than the grace period. The grace period is in Experiment Settings and is 5 minutes by default.
• A participant who arrives while the study is paused, full or not active. They see a page that says that the study is not available. The page for a full study also tells them to return the task to their recruitment platform.

Before you approve submissions, find these participants in the export.

Match Carrier to Prolific before you approve

In the export:

• participantId is the PROLIFIC_PID.
• studyId and sessionId are the STUDY_ID and SESSION_ID of Prolific.
• outcome and terminationReason show how each run ended: Completed, Screened Out, No Match, Attention Check Failed, Ended by Researcher or Dropped.

Before you approve, compare these with the codes that Prolific received.

Caution: the completion codes are in the study page. Anyone with the link can read them. Thus, approve by the export, not only by the code.

A participant whose Prolific ID is already in another of your Carrier studies gets the ID <id>_<study id>. Carrier records them and sends them back with this ID.