Payment for two-part study with mandatory and optional part

Dear Prolific Community,

I have a question regarding participant payment. We are designing an experiment which contains both a mandatory and an optional part. The idea is that participants get paid to complete a mandatory part and can then end the survey or continue for up to 10 minutes to generate a donation to a charity (which they can stop at any time).

My question is the following: Is it in line with Prolific’s fair pay policy to specify pay and duration of the study according to the mandatory part only if we clearly indicate in the instructions that after the mandatory part participants can either obtain the completion code or optionally continue for charitable purposes.

As I expect at least some participants to continue, the average reward per hour (which is calculated after completion of the study) will therefore probably be quite low, as those participants who voluntarily continue will push up the average time taken to complete the study.

I hope that this wouldn’t be an issue as measuring this additional charitable “effort” is the main goal of our study.


Hey Paul, great question!

Unfortunately, we don’t support studies which have a ‘voluntary participation’ aspect, because of the issues you’ve outlined. The study will be flagged as underpaying when advertised to new participants, so they might not get a very high response rate. Plus, participants will be quick to report it if they don’t read the instructions properly.

However, there are two potential workarounds:

  • One option could be to contact participants via the messaging system, after they have completed the mandatory part, with a link to the optional charity survey. If you make it clear in the message that it was an unpaid survey for charity and completely optional, I wouldn’t think we’d get any complaints. But, response rates may not be amazing, and I’m not sure if that fits in with your study design?

  • Another option would be to pay the minimum base rate (£5/hour), and then give participants the opportunity to donate part of a bonus payment. When the study is completed, participants will be paid the base salary after you approve submissions, then you would be able to apply bonus payments to individuals/charity according to their choices.

You’ll need to record Participant IDs within your survey, so you are able to match each submission with the appropriate bonus payment.

Does either option work for you?

Hi Josh,

Thank you for your reply and I appreciate your suggestions. Unfortunately, the second workaround is not ideal in our case, as our study was explicitly designed to measure voluntary effort/time investment rather than a simple donation decision.

Regarding the first workaround, I would be worried that the majority of people would ignore the message, and we would not get many responses. May I suggest one more workaround, which is along the lines of your suggestion.

• After the mandatory part, we have a page where we show participants the completion code and ask all of them to manually enter it at that stage. On the same page, we would also advertise the voluntary part but only allow participants to continue once they have inserted the code. So they would just need to switch tabs, enter the completion code and then can return to the survey. “Please ensure that you have entered the completion code, before continuing to the voluntary part, if you wish to do so”. There could be an extra “completion check” question, to make sure that nobody has skipped this. This way, the study should not show up as “underpaying”, but we also wouldn’t loose too many people due to a complete break of closing the survey and receiving a message with a totally “new” survey.

Do you think this idea might work?


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Actually, that could work :tada:

But some participants could be confused if they have to complete their submission on Prolific then return to the survey. I would strongly advise that you run a pilot study with this approach, and give us a heads-up when you launch it, so we can keep an eye out for mediation issues.

Easiest way to do this would be to email us from the support request form here!

Good luck with the study! Let me know how it goes :slight_smile:

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Hi Josh,

Thank you very much for your response and for your flexibility with the implementation of our study. We will definitely trial this study with a small number of participants and I will make sure that support is aware of it before we publish the study. If there are any issues, I will pause the data collection and make sure that none of the participants are unhappy.

Will keep you posted on how it goes :slight_smile:

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No problem Paul :slight_smile: Good luck!

Hi Josh,

Just writing to update you and anyone interested in this sort of design on how our pilot study went. In short, the compulsory/voluntary design worked well. We clearly indicated the end of the compulsory part by providing a link with the completion code and asking participants whether they wanted to continue or not. If no, they were automatically re-directed to Prolific with the completion code and their response was recorded. If yes, they were shown an additional page with the completion link and asked to confirm that they had entered the code before continuing. We also clearly stated that they would not be paid for the voluntary part. Participants were then able to continue the voluntary part for charity which they could exit at any time.

In our baseline condition, 25% of respondents chose to continue to complete the voluntary part in which they could generate a donation charity. However, this estimate is likely to be quite inaccurate as we ran the pilot with a small number of participants. We also slightly over-paid the compulsory part, so people may have been more willing to continue.

I’m so glad it worked! We’re always interested to see the new ways in which the platform is being used.

If another researcher on the forum wanted to use a similar set up, would you mind if I tagged you, and you could consult with them? :slight_smile:

And let us know the results from the full study!

Sure, anyone is welcome to get in touch :slight_smile:

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Hi, I am new to the platform so apologies if this is not the right place. I am looking to do something very similar to this - measure pro-sociality through someone being willing to give their time to donate to charity, and would love to understand how it worked for you, and how you did it? For example, did you mandate the charity the donation was made to? Did the participant know? What did you ask them to do in exchange for the donation, and did you ‘de-brief’ them afterwards?

Thank you!

Hi Gin

Welcome to the forums.

I think Aja and Elena also carried out a similar studies, mentioned in the following links

Depending on forum settings Paul may not be notified of your interest but his email address (according to a correspondence address on a journal article someone by of his name in his field) is pml44 at cam dot ac dot uk at which you may consider contact him directly should he not respond.


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Thank you so much!

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