When a sample is biased, some people or items are more likely to be chosen than others.
A group of 35 school children are surveyed. Is the sample likely to be biased?
Yes, because they may have different opinions to others in the school.
Jack selected 15 cards randomly from a pack of cards. Is the sample likely to be biased?
No, because the cards were randomly chosen and there is an equal chance that each card will be chosen.