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  • Mark Denson thinks this is interesting:

You can often get more revealing results if you allow the participants to choose some of the details of the task. It’s much better, for instance, to say “Find a book you want to buy, or a book you bought recently” than “Find a cookbook for under $14.” It increases their emotional investment and allows them to use more of their personal knowledge of the content.

Note

An investment by the tester (more than just their time) is crucial otherwise they're simply there for the reward which leads to them getting out of there as fast as possible and still keep the reward.