Question

Sue asked three of her friends after class if they thought the test they just finished taking was as easy as she thought it was. they all agreed that it was. she was surprised to find out the next day that, although she and her friends had indeed done well, a majority of the class had failed. why shouldn't sue have been surprised?

Answer 1

Her friends were problably just as smart as she was.

Answer 2

Consider the sample she used. She did not use a random sample, she selected her friends which introduces all sorts of biases. Her friends could all be studious or smart. Perhaps they are all friends because they are part of a study group? Or have similar academic abilities?