Four algorithm modifi cations, designed to address shortcomings in the C2 algorithm, were tested. The fi rst modifi cation tested was stratifi cation by weekdays versus weekend days. Although many methods have been used to adjust for differing counts by day of week (11) , these methods may require customization to specifi c datasets and a long data history (up to several years). Our simple method is to stratify the baseline days used to calculate μ and s t into weekdays versus weekend days. This stratifi cation is denoted the W2 algorithm. For example, a 7-day W2 baseline for weekdays contains the most recent 7 weekdays. For unstratifi ed and stratifi ed analyses, the 2 days immediately before the index day were excluded from the baseline, a standard practice for C2, to avoid contamination with the upswing of an outbreak.