While many health-related outcomes are measured on a continuous basis (age, blood pressure, virus particles per milliliter of blood), many common types of epidemiologic data analysis are performed using categorical data. Categorical data are data that exist in discrete groupings. Categories may be predefined, such as gender (male or female), while others may be defined by cutpoints in the data, such as hypertensive (blood pressure greater than or equal to 120/80) or normotensive (blood pressure lower than 120/80). This type of classification results in having only two categories, as opposed to the full range of blood pressure measurements possible. Because these types of data are so common, a specialized field of statistical techniques has been developed. This section documents the key approaches for categorical data ...

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