Motivation for pooling standard deviations by minimum intensity. A hypothetical noise distribution is given with higher noise at low intensities. Two sets of replicate spots (N = 3 arrays) that have the same average intensity are shown. However, example 1 produces a higher standard deviation of the logged ratio compared to example 2, because example 1 contains very low intensity measurements that fall into the noisiest range of the intensity scale. In this case, the minimum intensity would differentiate between these two examples while the average intensity would not.