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When you set up a study on Prolific, you have to estimate the completion time. The reward per hour shown before you collect any data is based on this estimate (must be at least 5 GBP / 6.50 USD per hour - read more about our pricing here).
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 £2.50 each. The reward per hour shown here is, therefore, £7.50/hr:
When you publish your study and receive submissions, the average completion time for your study may differ from your initial estimate. If the participants for my example study above actually took 30 minutes to complete on average, then the average reward per hour for this study would decrease to £5.00 per hour.
If the average reward per hour for your completed study falls below the minimum rate required (£5.00 / $6.50 per hour), you should send bonus payments to your participants to bring this up to at least the minimum rate. You will receive an email notification if this occurs, but you can also read more about how to do this here: hc/en-gb/articles/360009222573-Bonus-payments.