EPID 684

Spatial Epidemiology

University of Michigan School of Public Health

Jon Zelner

`[email protected]`

`epibayes.io`

**Global Clustering**

Population

*average*measure of spatial similarityImportance of effect assessed by scale of statistic relative to range or sampling distribution.

**Local Clustering**

Intensity of similarity

*relative*to surrounding areas.Can be used to identify

*hotspots*and*coldspots*.Meaningfulness often assessed in terms of how different they are from other small units in the same population.

\[G_{i}(d)=\frac{\sum_{j}w_{ij}(d)x_j}{\sum_{j}x_j}\]

Basically the proportion of all cases within a threshhold distance \(d\) of location \(i\).

Where:

\(d\) is the maximum distance to consider clustering

\(w_{ij}(d)=1\) if place \(i\) and place \(j\) are within \(d\) of each other, and 0 otherwise. (When \(i=j\), \(w_{ij}=0\))

\(x_j\) is the variable of interest

Yields an estimate of both strength of clustering and approximate statistical significance.

1.

Frazier AE, Hemingway BL, Brasher JP. Land surface heterogeneity and tornado occurrence: An analysis of Tornado Alley and Dixie Alley. *Geomatics, Natural Hazards and Risk* [electronic article]. 2019;10(1):1475–1492. (https://www.tandfonline.com/doi/full/10.1080/19475705.2019.1583292). (Accessed January 22, 2020)