# What is an outlier?

An outlier is defined as a value far from most others in a set of data. An outlier that is many standard deviations from the mean will have dramatic impact on the average. Outliers should be identified and cast out of the data set to obtain a more meaningful average. However, identifying a large number of outliers for a given gene can indicate true variability of the mRNA abundance – an equally valuable measurement. The outlier definition is a number (such as 1, 2 or 3) used by GPAP to identify and remove outliers to calculate the final average Log2 ratios within and across array’s replicates for each gene and is included in the Gene Summary Report . If the outlier definition is set at “2”, any Log2 ratio that is outside of 2 standard deviations of the mean is considered as outlier, is removed and the average is re-calculated. If the outlier definition is set at “3”, any Log2 ratio that is outside of 3 standard deviations of the mean is considered as outlier, is removed and the average