The mean count for each facility syndrome was calculated and categorized as follows: 0.5 to <2, 2 to <4, 4 to <6, 6 to <8, 8 to <10, 10 to <20, 20 to <40, and >40. Empirical distributions of the test statistic (e.g., number of SDs by which the observed count exceeds the expected value) were conducted separately for each dataset, algorithm, and mean count category; the 99th percentile value for each of these distributions was used as the cutoff value to defi ne an alert rate of 1%. For example, for the standard C2 algorithm in DoD data with mean count 4 to <6, a cutoff value of 3.9 was used because 1% of the facility-syndrome days had a test statistic >3.9. Because no attempt was made to fi nd and exclude real outbreaks from the data, these cutoff values defi ne an alert rate rather than a false alert rate, the latter being equivalent to 1-specifi city (12) .