in which f block represents the number of spots on one edge of the block, and f win is chosen by users from five options: 1, 3, 5, 7 and 9. Different windows can overlap with each other and go beyond the block edges. Let s denote signal intensities of spots within the local window; ProCAT uses two parameters to characterize the signal distribution of s: median (MED) and median absolute deviation (MAD):