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. That is, you would set up one Prolific study per time-point.
For each individual study on Prolific 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.
Please note that the reward rate for each individual study across a longitudinal study on Prolific must meet our minimum reward rate of £5 / $6.50 per hour.
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 step-by-step guide.
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.
You can invite your chosen participants to your follow-up studies using a custom allowlist.
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 custom allowlist. However, you will only need to run one study for the first part if your survey software supports allocation to conditions from one URL.
Additional points to consider
- Please note 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 time zone(s). We cannot guarantee that participants will take part at the specified time.
- 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 the above recommended set-up isn't going to work for your study, please get in touch to discuss alternative options by using the button at the bottom of this article.
Longitudinal study FAQs
We tend to see very low attrition (drop-out) rates in longitudinal studies if the requirements of the full study are made clear upfront, and the time between follow-up stages is not too long.
Drop out rates depend on a lot of factors. To name a few:
- How far apart are the different parts of the study?
- How generous are the rewards?
- How long do the different parts take?
We've had studies with 0% dropout rate and as high as 50%. A typical study would be somewhere in between these extremes. An independent study by Kothe and Ling found attrition of <25% over 1 year. Shorter longitudinal studies following best practices can expect better retention than this.
There are some steps you can take to minimise the attrition rate:
- You should clearly communicate your study information (i.e. expectations of participants, reward structure, time gap between the phases of your study).
- You could also screen for those with at least 20 previous submissions, to ensure you are obtaining active and committed participants. You can do this by applying the 'number of previous submissions' prescreener. Inexperienced participants are more likely to drop out.
- Paying a generous reward and offering bonus incentives to participants who complete all parts of your longitudinal study is also a good way to minimise attrition.
If your study consists of multiple waves you can wait until the final wave is complete before paying each wave provided that the waves are within 21 days. To do this, you would leave the submissions from the earlier studies as 'awaiting review' until you are ready to approve them. Submissions will be automatically approved after 21 days if they are still awaiting review at this point.
Please specify that payments will be delayed until completion of all parts in your study description, so that participants are fully aware of the requirements in advance of agreeing to take part.
If the waves are over a longer period all participants from each stage will need to be approved within 21 days.
If a participant on a custom allowlist has informed you that they are unable to see your study on their dashboard, please get in touch with our Support Team for further assistance using the button below.