Diluting a cancer study

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An often-cited study by researcher John Morgan found no cancer cluster in Hinkley, Calif. But the study looked for cancers throughout a census tract stretching over 137 square miles. The neighborhood contaminated with high levels of hexavalent chromium covered only one square mile.

The red area shows where water wells exceeded state standards for chromium. Former residents of 13 homes there sued PG&E and were featured in the film Erin Brockovich. PG&E bought and destroyed these homes by early 1993. The blue area shows the census tract used in the study. Most of the people living there were exposed only to low levels of chromium that are naturally found in this part of the Mojave Desert.