Ratio level data predominate on thematic maps. Ratio data are of several different kinds, including counts, rates, and densities. As stated earlier, counts (such as total population) are whole numbers representing discrete entities, such as people. Rates and densities are produced from pairs of counts. A rate, such as percent population change, is produced by dividing one count (for example, population in year 2) by another (population in year 1). A density, such as persons per square kilometer, is a count divided by the area of the geographic unit to which the count was aggregated (e.g., total population divided by number of square kilometers). It is conventional to use different types of thematic maps to depict each type of ratio-level data.