Hello everyone! I have a question about “first language” filters. I want to recruit native English speakers only, so I selected “English” as an appropriate filter. However, after recruiting 4 initial participants and checking their data, it seems that only 1 of them is actually English (I included several demographics questions that do not correspond to what these people answered in your screening questionnaire, and I’m also messaging them). Is there any way I could potentially improve the screening so only native English speakers get my invitation? My experiment is 1 hour long, and rejecting people who spent so much time on it feels horrible. I clearly say “native speakers of English only”, but I’m guessing since they receive an invite from Prolific, they don’t see a problem with completing the study.
Hey Magdalena, welcome to the community! It’s great to have you aboard 
We run checks on participants to ensure that their language abilities are as good as they claim. But some people do manage to evade these checks, and I’m sorry that you’ve experienced that!
To help combat this, we’ve enabled the reporting of participants in-app. You can read about that here. Once you report them, we will investigate and take actions to safeguard data quality.
Thanks for letting us know! 
Hi Magdalena
Another thing you might try is, instead of, or in addition to
using the “first language” filter (since “first language” has
a subjective component and someone living in the UK or
US might claim that their “first language” is English, since
it is the first language of their nation, when they use
another language at home) you could also use the
" English speaking Monolingual > I only know English"
filter, which would sort out bilinguals who are claiming
that their English is “first” though weak.
There is also the " Were you raised monolingual?"
filter.
Further, in the “Basic demographic variables >
mono/multi cultural” filter, but you may wish to
accept multicultural native English speakers.
Tim