The accuracy of expected value calculation was evaluated by using mean absolute residuals. For lower residuals, expected values are closer to observed values than they are for higher residuals. Similarly, the expected value calculation is more accurate for lower residuals than for higher residuals. For the DoD data, lower residuals were seen with stratifi cation (W2) and the rate algorithm: mean residual 4.2 for unstratifi ed count algorithm versus 2.2 for stratifi ed rate algorithm (Table 2) . For the hospital ED data, residuals were lower for the rate algorithm, and stratifi cation had a minimal effect. Varying the baseline duration and minimum SD had no effect on the accuracy of expected value calculation (data not shown).