mean

The mean, or average, is the total of a set of values divided by how many values there are. For 6, 12, 9, 15 and 3, the total is 45 and there are 5 values, so the mean is 45 ÷ 5 = 9. The mean uses every value, which makes it sensitive to an outlier: one very large or very small number pulls it toward that number. That is why a data question sometimes asks for the median instead — the mean of 1, 2, 3, 4 and 100 is 22, but the median is 3. The common error is to stop at the total and never divide by the count, reporting 45 as the mean. Read the unit carefully: a mean of daily counts is reported "per day". The mean is one of the three measures of a data set's center, with the median and the mode.