I have two questions about pre-screening:
Dear Layelderman
I believe it screens for residence in the US or UK and includes foreign nationals.
You can not add a prescreener if you have selected representative sample. In order to hit all the demographics, Prolific must use all of the database of participants so if you reduce them using a prescreener then may not be able to get the participants you need.
To use a presscreener, which is free of charge, you must select “Standard Sample” and then “Add screener” (Please see below)
You will then see the number of active participants change (be reduced) from the full c. 122,000 to show you how many active participants there are that match the combination of screeners that you have added. They are added only with an AND logic. For OR logic you would need to create multiple studies.
It is okay to try out the screeners. Nothing is communicated to the participants (nor to Prolific afaik) till you launch the study.
I hope that helps 
Tim
Timtak
So far the available pre-screens would not work. Also, for my study, I would need just a single question about foreign national status AND a representative sample. I am not seeing how the 2 study approach would work with a representative sample, unless I can capture nearly all possible participants (tens of thousands) that would meet the eligibility. Even then, I couldn’t set the custom allow list pre-screener question and still have a representative sample (as you stated above).
Could I ask participants to voluntarily screen themselves? This way I am not forcing them out of the survey based on their answer to a question, I’m just asking them to voluntarily stop the survey if they are a Foreign National.
Something like:
Foreign Nationals are not eligible to participate in this study. If you are a foreign national, please voluntarily withdraw at this time. Please return this submission on Prolific by selecting the “stop without completing” button. If you proceed, you are acknowledging you are not a Foreign National.
Dear Layelderman
Please call me Tim.
I am sorry to hear the prescreeners do not work for you.
I am sorry to say that it is not, as far as I know, currently possible to apply screeners to representative samples. That may be why the prescreeners are not working for you.
I think you are right about the two study approach would not work for you either.
As you correctly point out, you could not use a custom allowlist with a representative sample either.
One possibility is to create your own representative sample by creating a lot of studies (for each gender x age group x race) based on the target population statistics (a link to which is given on the representative sample FAQ page). Then you could add a nationality screener to each of these studies. This method would require a lot of clicking but it would not incur any additional cost.
CORRECTION
The prolific rules say "It is also not appropriate to put the screening criteria in the study description alone. " so asking participants not to take part is also not an option.
The prolific rules stipulate that we are not allowed to put screeners in our surveys.
You may also wish to submit a feature request so that it is possible to exclude foreign nationals from representative samples on the forum below.
Someone else raised the issue of whether representative samples are limited to nationals or not so it is a question of concern and and may receive votes.
I am sorry I can’t be of more help.
Tim
