updated 3/11/2005 11:56:08 AM ET 2005-03-11T16:56:08

Methodology
To look at the cleanliness of the nation's leading fast food chains, Dateline NBC used trade publications and information from the chains themselves to determine which operate the most fast food restaurants in the United States. It then hired a company that had lists of all of the locations for those chains. It randomly selected 100 restaurants for each chain. Dateline NBC then called state, county and local health departments across the nation to obtain health department inspection reports covering Nov. 1, 2003 to Oct. 31, 2004. This added up to more than 2,400 inspections.

Dateline NBC then analyzed these reports for critical violations -- things like evidence of vermin, food not kept at proper temperatures to ward off bacteria. These are violations that health experts say that, if not followed, may make people sick from food-borne illnesses. We then examined the data for violations found during routine inspections. These are generally unannounced visits by local government health inspectors.

Dateline NBC then ranked each chain based on the average number of violations. Within the list, with guidance from statisticians, Dateline NBC then examined the averages. The chains fell into three categories. Within each category, the differences among the chains are not statistically significant. However the groups are statistically significant from one another. So, for instance, Taco Bell and Jack In The Box had average violations that were not significantly different statistically. But they were significantly different from McDonald’s and the others at the top of the list. The statistical groups are represented in different colors in the chart.

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