Division of Communicable Disease Control, California Department of Health Services, Sacramento, CA 94234-7320
We thank Drs. Mulla and Cole (1) for their interest in and letter about our article (2). The confidence intervals presented in table 2 (2) are incorrect because of an error that was made in calculating the variance. An abbreviated version of table 2 with the corrected confidence intervals is presented below. Mulla and Cole are correct in that there are statistically significant differences between the rates that were not apparent by examination for overlap of the confidence intervals in the original table 2. However, most of the rates are significantly different from one another when taking into account the newly calculated confidence intervals. The results below agree for the most part with those obtained using a direct test of the difference in two proportions, but there were two hospitalization rates with overlapping confidence intervals that were significantly different by this test: 1517 years of age versus 1839 years of age (chi-square = 5.4, p value = 0.027); and males versus females (chi-square = 7.34, p value = 0.01). The large sample sizes in the present study provide one explanation for why the differences between so many of the rates are statistically significant, as even small differences may be statistically significant if the sample size is sufficiently large (3).
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