You want to know the type of music listened to by 15- to 20-year-olds. Which of the following surveys is biased?
surveying people 15 to 20 years old leaving a concert,
surveying people 15 to 20 ...
... years old on the street,
or surveying everyone at the local music store.
Surveying people 15 to 20 year olds leaving a concert, is a biased survey. The statistics wouldn't be good because all the concert goers like that specific type of music if they went to that concert.
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