when designing manipulative experiments , scientists have to be careful to avoid bias.
Bias occurs when a scientist has some preconceived ideas or preferences Concerning what she's testing.
These ideas may influence how she sets up the experiment, how she collects the data,
and how she interprets the data.
To avoid this bias , a scientist can set up a blind experiment,
in which other scientists set up a control group and a manipulated group and don't inform the scientist who's actually observing the experiment which one is which.