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It is so common place because as an average if 100 people had to participate on a sample, than approximately 95 of them will be outside of the mean. Let’s say that we ask 100 random students to choose 5 numbers from 1 to 100. If we draw a bell shaped curve, the mean will be the number 50. One of the students may chose, 2, 37, 78, 90 and 100. While another may chose 1, 5, 46, 56 and 72. The mean of the student #1 was 60.8 which is outside of the 50 mean. Student #2 had a 36 mean which is also outside of the mean 50. The logic is that if we had all 100 students do this sample, approximately 95% of them will not have a mean of 50. The first time we do it with 100 students we may have a 90% confidence interval. This is strictly hypothetical and the outcomes could be different every time. But 95% is the most probable number that we will get if we do this sample indefinitely.