This feature is now available on https://app.prolific.ac/ Select the option to "Increase Places" from the blue Action menu.

The study's pricing will automatically re-calculate when you do this.

Note that you can only increase places for studies that are still active. If you want to collect more participants for a study that is already completed, you can duplicate the study and add a 'Previous Studies' screener: http://help.prolific.ac/creating-prescreening-eligibility-requirements/how-can-i-exclude-participants-who-have-previously-participated-in-my-study.

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