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Am J Epidemiol 2004; 159:104-105.
Copyright © 2004 by the Johns Hopkins Bloomberg School of Public Health

LETTERS TO THE EDITOR

TWO AUTHORS REPLY

Rosalie Trevejo and Mark Starr

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: 15–17 years of age versus 18–39 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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  		TABLE 2. (Revised) characteristics of Salmonella patients in California: reported cases and hospitalizations
 
In this table, the confidence intervals were selected for presentation because they allow examination of the individual rates as well as comparisons between the rates while obviating the need to present every possible comparison.

REFERENCES

  1. Mulla ZD, Cole SR. Re: "Epidemiology of salmonellosis in California, 1990–1999: morbidity, mortality, and hospitalization costs." (Letter). Am J Epidemiol 2004;159:104.[Free Full Text]
  2. Trevejo, RT, Courtney JG, Starr M, et al. Epidemiology of salmonellosis in California, 1990–1999: morbidity, mortality, and hospitalization costs. Am J Epidemiol 2003;157:48–57.[Abstract/Free Full Text]
  3. Hennekens CH, Buring J. Analysis of epidemiologic studies: evaluating the role of chance. In: Mayrent SL, ed. Epidemiology in medicine. Boston, MA: Little, Brown, and Company, 1987:243–71.
  4. Office of Statewide Health Planning and Development. Patient discharge data file documentation: public versions A and B. Sacramento, CA: California Department of Health Services, 1998.

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Am. J. Epidemiol., January 1, 2004; 159(1): 104 - 105.
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