An outlier is a data value or an observation point which is far or distant from the other observation. It is is normally much smaller or larger than most of the other values in a set of data.
Outlier occurs as a result of variability in the measurement or as a result of an experimental error. Outliers affect statistical analysis. They affect the mean value of the data, they have a little effect on the median or the mode of a given set of data.
