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Letters to the Editor
February 2005

On the Use of Clusters to Determine Environmental Influence on Disease

Arch Neurol. 2005;62(2):331. doi:10.1001/archneur.62.2.331-a

In the article titled “Clustering of Parkinson Disease: Shared Cause or Coincidence?,”1 3 clusters of Parkinson disease (PD) in Canada were analyzed to come to the conclusion that there is an important role for environmental causation. We disagree with this conclusion since the authors examined outliers while ignoring the population as a whole. Our conclusion is the opposite: that such clusters, while rare, are to be expected in a country the size of Canada, and therefore no conclusion can be drawn regarding environmental factors.

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