The owner of a laboratory wants to keep sick leave as low as possible by keeping employees healthy through disease prevention programs. Many employees have contracted pneumonia leading to productivity problems due to sick leave from the disease. There is a vaccine for pneumococcal pneumonia, and the owner believes that it is important to get as many employees vaccinated as possible. Due to a production problem at the company that produces the vaccine, there is only enough vaccine for half the employees. In effect, there are two groups; employees who received the vaccine and employees who did not receive the vaccine. The company sent a nurse to every employee who contracted pneumonia to provide home health care and to take a sputum sample for culture to determine the causative agent. They kept track of the number of employees who contracted pneumonia and which type of pneumonia each had. The data were organized as follows:

• Group 1:Not provided with the vaccine (unvaccinated control group, N = 84)
• Group 2:Provided with the vaccine (vaccinated experimental group, N = 84)

In this case, the independent variable is vaccination status (vaccinatedversusunvaccinated). The dependent variable is health outcome with two levels:

• Contracted pneumoccal pneumonia;
• Did not contract pneumonia.

The company wanted to know if providing the vaccine made a difference. To answer this question, they must choose a statistic that can test for differences when all the variables are nominal. The ?²statistic was used to test the question, "Was there a difference in incidence of pneumonia between the two groups?" At the end of the winter, Table 1 was constructed to illustrate the occurrence of pneumonia among the employees.

Table 1. Results of the vaccination program.

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