Validation of the algorithm-derived Ki-67 proliferation index (PI). The software algorithm counted an average of 10.9% more positive nuclei (A) and 4.8% more negative nuclei (B) for each field. However, the PI for each field was very similar to the algorithm, calculating on average a higher PI of 3.5% (C). The algorithm and manual counts for each field are plotted against each other, revealing a strong positive Pearson product-moment correlation (Pearson r = 0.99, df = 28, P < .001) with a slope of 1.04 (D).