When positive spots are likely to be close to each other, Pro-CAT uses box plots to examine and remove possible outliers from the surrounding window [24] . Let Q 1 be the lower quartile (25th percentile) and Q 2 be the upper quartile (75th percentile); the difference between Q 1 and Q 2 is termed interquartile range ΔQ. A spot (i', j') is then defined as an outlier if its signal: