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The confidence interval measures the reliability of a statistic, such as an opinion poll. For example, if 40% of the sample interviewed declare to choose a certain product, it can be inferred with a level of confidence of 99% that a percentage between 30 and 50 of the total consumer population will be expressed in favor of that product. From the same sample interviewed, with a 90% confidence interval, it can be assumed that the percentage of opinions favorable to that product is now between 37% and 43%.