To run a longitudinal study on Prolific - i.e. where you require participation from the same individuals over multiple time-points - you can set up multiple Prolific studies and invite the same participants back to each one using a Custom Whitelist. That is, you would set up one Prolific study per time-point.
For each individual study, you should set the completion time and reward per participant to reflect what participants will be required to do within that stage of the experiment only. So, if you're running a 2-part longitudinal study with an overall reward of $3.00 and each part takes 10 minutes to complete, you would set this up as follows:
- Study 1: Estimated Completion Time 10 minutes, Reward per Participant $1.50
- Study 2: Estimated Completion Time 10 minutes, Reward per Participant $1.50
When participants have completed both parts of the study, you can approve both of their submissions so that they are paid the full $3.00 reward. Detailed instructions for setting up a longitudinal study are provided below.
The first study in the series
You don't have to do anything particularly different when setting up your first study, you can mostly follow the instructions in our basic study tutorial.
It's especially important that you're recording participant IDs in your survey software for a longitudinal study, so you can follow-up with these participants for the subsequent studies. If your survey software supports it, recording the IDs automatically using URL parameters is strongly recommended.
You'll want to make sure that you explain the full structure of your study clearly in the Study Description, especially if payment is contingent upon participation in the follow-up stages.
The follow up studies
When the first study is complete you can get the participant IDs of the relevant participants by exporting your Prolific data. You'll need these IDs when setting up the rest of the studies in the series.
After filling out all the required information (as you would for any other study), go to the "Audience" section. Click on "Custom prescreening" here, and search for the "Custom Whitelist" screener. Add the participant IDs of the participants you want to be able to take part in this follow up study.
ONLY these participants will be able to take part in this study. They will automatically be emailed with an invitation to take part when the study is published, though you can also send custom reminders using our messaging system.
If you have multiple conditions and you want to keep these conditions consistent between studies, you will need to set up a separate follow-up study for each condition and add only those participants you want to be eligible to the Whitelist. However, you will only need to run one study for the first part if your survey software supports allocation to conditions from one URL (e.g. Qualtrics Randomizer).
To generate the Whitelist, it is best to obtain the IDs from your Prolific data export rather than from your survey software. Prolific IDs recorded in your survey may contain typos, and any invalid IDs included in the list will be rejected by our system.
Additional points to consider
Please note that we have an auto-approval mechanism in place which will activate after 21 days, which means that participants' submissions will be automatically approved 21 days after they are completed.
If you want your participants to complete the follow-up studies at particular times, please make sure to specify this clearly in your study description, including the times zone(s). We cannot guarantee that participants will take part at the specified time.
We tend to see very low attrition (drop-out) rates in longitudinal studies if the requirements of the full study are made clear up front, and the time between follow-up stages is not too long. Click here for some helpful tips about how you can further minimise attrition across multiple time-points.
When participants have a genuine reason for dropping out of a longitudinal study, you may wish to consider offering them partial compensation or approving their submissions in the parts they have completed. Please show consideration towards participants when reviewing submissions and deciding on a fair level of compensation.
If you have any questions about running a longitudinal study, feel free to get in touch via email@example.com.