Which of the following is NOT a method described for depicting spatial distribution of disease frequency?

• A. Areal maps (within defined boundaries)
• B. Spot (or dot) maps
• C. Overlay mapping (superimposed spatial distribution maps)
• D. Choropleth maps

Answer: (D)

Depicting spatial distribution of disease frequency relies on mapping approaches that translate case data into a geographic form. Areal maps show values within defined boundaries like counties or districts, giving a picture of how a disease burden varies across areas. Spot maps place a dot for each case, or for each unit of risk, to visualize concentration without aggregating to a boundary. Overlay mapping layers multiple spatial datasets on top of one another, so you can compare patterns and assess relations between disease and other factors such as population density or environmental exposure. Choropleth maps color geographic areas according to a statistic (such as rate or case count) to convey differences across regions.

Choropleth maps are a standard way to depict disease frequency by geographic units, using shading to represent the magnitude of the statistic. Because of that, the option describing a choropleth map as not a method used for spatial depiction doesn’t fit with common practice. In practice, all four approaches described are valid methods for illustrating spatial distribution, with choropleth maps adding a color-graded, area-based view.