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Introduction to Categorical Data Analysis Procedures

Randomized Experiments

Consider a randomized experiment in which patients are assigned to one of two treatment groups according to a randomization process that allocates fifty patients to each group. After a specified period of time, each patient's status (cured or uncured) is recorded. Suppose the data shown in Table 5.5 give the results of the experiment. The null hypothesis is that the two treatments are equally effective. Under this hypothesis, treatment is a randomly assigned label that has no effect on the cure rate of the patients. But this implies that each row of the table represents a simple random sample from the finite population whose cure rate is described by the column marginal totals. Therefore, the column marginals (58, 42) are fixed under the hypothesis. Since the row marginals (50, 50) are fixed by the allocation process, the hypergeometric distribution is induced on the cell frequencies. Randomized experiments can also be specified in a stratified framework, and Cochran-Mantel-Haenszel statistics can be computed relative to the corresponding multiple hypergeometric distribution.

Table 5.5: Two-Way Contingency Table: Treatment by Status
  Status  
 TreatmentCuredUncuredTotal 
 1361450 
 2222850 
 Total5842100 

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