One great tip to reduce bias

Don't use words that are on your design UI or prototype. Participants will just match the keywords you give them with the exact same words on your website or prototype.

For example: Your working on an insurance website where customers can manage their insurance. You want to test how easy it is for customers to find their Auto Insurance ID Cards that they put in their cars in case they get in an accident or get pulled over by the police. Do not say, "Show me how you would find and print your auto Insurance ID Card for your Jetta in a usability study" as the scenario for study. Instead say "Show me how you would find and print out the document you need as proof that you have Auto Insurance that you will put in your car in case you get in an accident or get pulled over by the police".