biased

This is a property of a statistical sample which means that the sample isn’t representative of the whole statistical population. A biased sample is unreliable as it means that some results are more likely than others.

For example, John wants to find out which supermarket do people in a specific town prefer. In order to do this, John intends to stand outside one supermarket only, and ask the following question:

Do you agree that this supermarket is the best in town?

This type of question in a questionnaire is biased.