Question

How do u improve your experiment and prevent bias

Answer 1

Considering it's talking about bias, I believe it's referring to more of a "social" experiment-- not a laboratory one-- so contamination won't be a problem. To improve your experiment, you need to have a wide range of test subjects, and a large number so you can get more accurate results. And to prevent bias in an experiment, you might want to do a double blind study. 

Answer 2

No, no, no contamination in your experiment. Cut off all sources of possible bias. 

Always be as precise as you can.

That's just to name two ways.