reading:讲的是概率统计里面的sampling error. when people want to drawconclusions about a large group, sometimes, the group is so large that it isimpossible to analyze it. So a sample is introduceD、When a sample is accurate,it can represent the whole group.But when it is inaccurate, sampling errorwould occur.listening: a researcher conducted an experiment to research the amount of fish ofdifferent sizes -- big fish, middle size fish, small fish.Because there are manyfish in the lake so he wants to select a sample from the lake by his net.Butthere are just big fish and middle size fish in his net. He is surprised thatthere is no small fish in the lake.But he then figures out that this is becausesmall fish has escaped from his net.