Difference ratio vs. expression level. Each point represents the ratio of the differences in expression level of a microRNA and the mean of all microRNAs against the mean of the CT values for that particular microRNA across all samples. The difference ratio is calculated by dividing the difference of a miRNA's expression in a particular sample by the difference between that sample's mean and the overall mean. For each miRNA a vector of difference ratios is calculated with one value for each sample. On the figure the y-axis represents the mean difference ratio for a particular miRNA. The ratio of this difference increases with increasing CT, demonstrating that lowly expressed miRNAs are more sensitive to fluctuations in the mean.