Since there is no reference to an absolute value, we define overexpression as a dot that has significantly higher intensity than dots in its locality. Since an overexpression is a statistical outlier, we measure the degree of spread of each dot from the local centre of distribution in terms of pixel intensity. The median absolute deviation (MAD) is calculated for each locality. Each dot of interest will yield an absolute deviation from median which is normalised with respect to its local MAD. As such, the local gain component is eliminated. The result is the number of MADs from median which is reported as a relative quantification of expression levels. Since the local offset and gain components were removed, the number of MADs from median can be used to compare levels of expressions across arrays from different samples and time points.