A dot distribution map places small point symbols over a given space to indicate the distribution of a given phenomenon. The location of each dot may represent the actual location of a single instance, as in the map made by Dr. Snow during the 1854 Broad Street cholera outbreak, where each dot represented one death due to cholera. Alternatively, aggregate statistical data (the same used in choropleth maps) may be mapped by randomly placing dots within each aggregation region (e.g., country, state, county) to show the general density of instances; this latter form is usually called a "dot density map".