Essay: The primary threats of validity
In most instances, the researchers are led by a specific objective to conduct their research. The experiments they carry out are meant to clarify what they already know theoretically to prove whether it is true or false. These people may dwell much on what they already know which can make them make controversial observations and eventually head to false conclusions that makes their research invalid.
For instance, if a person had earlier been informed that a certain amount of substance gives this observation if he/she is biased, they might end up making wrong observations in order to match with their expectations.