When you set up a study on Prolific, you have to estimate the completion time for your study. The reward per hour shown on the study creation page before you collect any data is based on this estimate. We have a minimum reward rate of £6 / $8 per hour.
For example, you will see in the following screenshot that the estimated completion time for my study is 20 minutes, and I want to pay my participants £3 each. The reward per hour shown here is, therefore, £9/hr:
When you publish your study and receive submissions, the average completion time for your study may differ from your initial estimate. The means that the average reward per hour may be higher, lower or the same as your initial reward rate. For example, if the participants in study above actually took 30 minutes to complete on average, then the average reward per hour for this study would decrease to £6 per hour.
If your average reward per hour drops below our minimum reward rate, or below your initial reward rate, you will be prompted to increase the amount paid to participants. There is further guidance on this here: How do I resolve underpaying studies?