The third modifi cation tested was adjustment for total daily visits. For the adjustment procedure, we used a formula in which n 0 = count of visits on the index day for the chosen syndrome (e.g., visits for the respiratory syndrome), and d 0 = the total number of facility visits on the index day, including visits that were both assigned and unassigned to any of the 11 syndromes. Σn i = total syndrome visits summed for all i baseline days. Σd i = total facility visits summed for all i baseline days. The formula for the adjusted expected value was e 0 = d 0 × Σn i /Σd i , which differed considerably from the mean of the n i if d 0 was high or low. Fewer visits for a given syndrome were thus expected on a day when the facility had fewer total visits. The estimated adjusted SD, s 0 , was taken as the mean absolute value of (n i -d i × Σn i /Σd i ) over i baseline days; that is, s 0 = Σ (abs (n i -d i × Σn i /Σd i ))/i. The test statistic adjusted for total visits was (n 0 -e 0 )/s 0 , analogous to the C2 statistic (n 0 -μ)/ s t , where μ and s t are the mean and SD of n i , the counts on baseline days. In the discussion below, we refer to this adjustment as the rate algorithm.