Choropleth Map Design for Cancer Incidence, Part 1 - Preventing Chronic Disease: January 2010: 09_0054

Volume 7: No. 1, January 2010

TOOLS AND TECHNIQUES
Choropleth Map Design for Cancer Incidence, Part 1

Thomas B. Richards, MD; Zahava Berkowitz, MSPH; Cheryll C. Thomas, MSPH; Stephanie Lee Foster, MPH; Annette Gardner; Jessica Blythe King, MPH; Karen Ledford, CTR; Janet Royalty, MS
Suggested citation for this article: Richards TB, Berkowitz Z, Thomas CC, Foster SL, Gardner A, King JB, et al. Choropleth map design for cancer incidence, part 1. Prev Chronic Dis 2010;7(1).
http://www.cdc.gov/pcd/issues/2010/jan/09_0054.htm. Accessed [date].

Abstract
Choropleth maps are commonly used in cancer reports and community discussions about cancer rates. Cancer registries increasingly use geographic information system techniques. The Centers for Disease Control and Prevention’s Division of Cancer Prevention and Control convened a Map Work Group to help guide application of geographic information systems mapping techniques and to promote choropleth mapping of data from central cancer registries supported by the National Program of Cancer Registries, especially for planning and evaluation of comprehensive cancer control programs. In this 2-part series in this issue of Preventing Chronic Disease, we answer frequently asked questions about choropleth map design to display cancer incidence data. We recommend that future initiatives consider more advanced mapping, spatial analysis, and spatial statistics techniques, and include usability testing with representatives of state and local programs and other cancer prevention partners.


Introduction
Maps are an effective tool for cancer control planning and evaluation (1-3). Data displayed on a map allow users to visualize spatial relationships and draw attention to areas of importance. Maps can be used to identify boundaries of complex geography, display rates for specific areas, reveal geographic patterns, and suggest questions for research (eg, what is the spatial relationship between cancer rates and risk factors such as socioeconomic status?) (1).

The National Program of Cancer Registries (NPCR), Division of Cancer Prevention and Control (DCPC), Centers for Disease Control and Prevention (CDC) supports state central cancer registries (CCR) in the collection of high-quality cancer incidence data (4). An increasing number of these registries assign geocodes (eg, latitude and longitude coordinates) to residential addresses of people with incident cases (5,6). These geocoded cases can be used to develop maps of cancer incidence rates and as part of spatial statistical analyses (7).

Choropleth maps are a common starting point for mapping cancer incidence. DCPC convened a Map Work Group to develop guidance for the design of choropleth maps and to promote mapping of NPCR-supported CCR cancer incidence data. Choropleth maps of cancer incidence rates assign colors to rate categories and then fill the area in the geographic units of interest (eg, states, counties, census tracts) with the color corresponding to that unit’s rate (8). The National Cancer Institute (NCI) and CDC state cancer profiles Web site provides good examples of choropleth maps (9). Many more advanced mapping methods exist, but these methods typically require investment in additional software or training for state program staff (7,10,11).

This 2-part series summarizes Map Work Group responses to common questions about choropleth map design. In this article we discuss the purpose of the map, geographic units of analysis, cancer sites, age-adjusted rates, rate ratios, and reliability. In Part 2 we discuss suppression rules to protect the privacy and confidentiality of cancer patients; questions related to mapping cancer stages, rates, and percentages; classes for map display; comparing maps over time; map color schemes, labels, projections, and output media; and limitations in interpretation (12).

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Preventing Chronic Disease: January 2010: 09_0054