Glossary of Terms

The terms below are defined and explained in the main article.

I include the definitions below for the purpose of convenience.


A "cluster" is defined for a list of 15 states with the highest cancer rates, although sometimes the list may be longer as explained below.

A cluster exists if the following three conditions are met:

  1. At least 80% of states in the list have shared borders, e.g. 12 out of 15 (80%), or 14 out of 17 (82%).
  2. The shared-border ratio is 100% or higher.
  3. A cluster may contain two (or more) disconnected groups of states, but a group has to contain at least 3 states with shared borders to be part of a cluster.

The above definition of what constitutes a cluster is partly common sense but partly arbitrary, since there is no scientific definition of what constitutes a "cluster." However, the definition is consistent with how the CDC defines cancer clusters.

In situations where states beyond the "15 states with highest cancer rates" have identical cancer rates to the 15th state, those states are included in the investigation. For example, in the list of states with highest cancer rates, if the 15th, 16th, and 17th states all have identical rates, then a cluster is searched in a list of 17 states (instead of 15).

Contiguous USA

Since this study was concerned with looking for shared borders among U.S. states, I used cancer data from states in the contiguous USA, consisting of the 48 adjoining U.S. states plus Washington, D.C. (District of Columbia).
For the purpose of fluidity of discussion, the District of Columbia is counted as a state since it reports it's own data and is not part of any state.


A "record" is defined as the distribution of age-adjusted cancer rates (incidence or mortality) for a specific gender and race/ethnicity across the U.S. during the period 2009-2013. As an example, the distribution of colon cancer mortality rates of Hispanic women for the period 2009-2013 across the U.S. is a single record.

Shared-border ratio

The shared-border ratio is the number of shared borders divided by the number of states, and is expressed as a percentage:

Shared-border ratio [%] = (Number of shared borders * 100) / (Number of states)

As an example, 15 shared borders among 15 states is a shared-border ratio of 100%. If there are 24 shared borders among 15 states then the ratio is 160%. More specifically, the four states Minnesota, Iowa, Wisconsin and Illinois have five shared borders among them, or a shared-border ratio of 125%.