To test these modifi cations, 2 datasets were used: records of Department of Defense (DoD) facility fi nal diagnoses for September 2004-November 2007 and records of hospital emergency department (ED) chief complaints for March 2006-November 2007. The DoD data consisted primarily of data from outpatient clinics; however, ≈15% of the visits in this evaluation were from patients seen in emergency facilities and cannot currently be differentiated in the BioSense System. We studied the 11 syndrome groups designed to be indicative of infections resulting from exposure to pathogens plausibly used in a bioterrorist attack (4). The DoD data consisted of daily counts of patient visits with International Classifi cation of Diseases, 9th Revision (ICD-9)-coded diagnoses categorized into the 11 syndrome groups. The hospital ED data consisted of freetext chief complaints, which were fi rst parsed for a specifi ed set of keywords, abbreviations, and misspellings and then categorized into 10 of the syndrome groups (1 syndrome, specifi c infection, was used for diagnosis but not for chief complaint data). Some ICD-9 codes and chief complaints may be included in >2 syndromes. However, counts of different syndromes were analyzed separately, not added together, and therefore are not double-counted in the analyses. For both datasets, we analyzed counts aggregated by facility. We included facility-syndrome combinations that had mean counts >0.5 over all facility-syndrome days in the study period. Many DoD clinics are closed on holidays. Therefore, for the DoD data, 11 days (days on which federal holidays are observed and the day after Thanksgiving) were recoded as weekend days for purposes of stratifi ed algorithm calculations (5) . Because hospital EDs typically are open on these holidays, no recoding for holidays was performed for this dataset.