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FIGURE 19-1: Yearly data on fatal highway accidents in one city.

Interpreting the Poisson regression output

After you follow the steps for running a Poisson regression in the preceding section, the program

produces output like that shown in Figure 19-2, which is the Poisson regression output from R’s

generalized linear model function (glm).

FIGURE 19-2: Poisson regression output.

This output has the same general structure as the output from other kinds of regression. The most

important parts of it are the following:

In the coefficients table (labeled Coefficients:), the estimated regression coefficient for Year is

1.3298, indicating that the annual number of fatal accidents is increasing by an estimated 1.33

accidents per year.

The standard error (SE) is labeled Std. Error and is 0.3169, indicating the precision of the

estimated rate increase per year. From the SE, using the rules given in Chapter 10, the 95 percent

confidence interval (CI) around the estimated annual increase is approximately