Drop out rates depend on a lot of factors (as I'm sure you're aware); 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 10 previous submissions, to ensure you are obtaining active and committed participants. Inexperienced participants are more likely to drop out.
* Pay 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 - you can read about how to do this here: Bonus Payments
Kothe, E. J., & Ling, M. (2019, September 6). Retention of participants recruited to a one-year longitudinal study via Prolific. https://doi.org/10.31234/osf.io/5yv2u