Participants run the study longer than estimated time

Not long ago we started a study with 2 hours 10 minutes as an estimated time of completion. That time was chosen based on a previous similar study with 100 participants and a pilot with 7 participants. However, in the study we are currently running we experience participants taking up to 1 hour 30 minutes longer to complete it. That in its turn forces us to increase the payment rate. Due to our previous experience it is hardly possible to take that much time to do the study. We wonder whether it happened in other studies and how we can improve the situation. Any advice will be very welcome.

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