Tills on the Allegheny Plateau in northeastern Ohio and northwestern Pennsylvania can be differentiated on the basis of percentages of sand, silt, and clay. However, for comparing size analyses of tills from very large populations it is convenient to reduce each analysis to a single number. The median size is the most useful single number; however, it can be determined only from a detailed analysis, which is very time consuming. Plotting the percentage of sand and clay on a rectangular graph and dividing the area into numbered narrow "zones" provides a series of numbers called "size factors" which serve the same purpose as median diameters. An approximation to median size is obtained by reference to a chart.