In math, outliers are observations or data points that lie an abnormal distance away from all of the other values in a sample. Outliers are usually disregarded in statistics because they can skew calculations and cause the final statistical analyses to be invalid.
Before determining if a data point is an outlier, the normal data point value range must be determined. To do so, a random sample of a population must be taken, and the data points must be ordered from smallest to largest. The normal range for the data set will include the majority of the points, while the outliers will be a lot smaller or a lot larger than any of the data points.