Figure 4.

Statistical process control chart* of the percent of visits with a positive rapid influenza test in UUPCRN clinics during the 2008-09 season. *Circled portion of the graph represents part (Dec 25–Jan 27) of the prealert period and includes the early alert signal. The signal occurred on Jan 26 (data point that did not return to baseline following a 3 sigma exceedance), with the data download on Jan 27, making the latter date the signal available to the analyst. Symbols used in the graph are A—designates a data point exceeded the upper control limit (3 sigma), C—designates the 9th data point in a row above the center line, and D—designates the 9th data point in a row below the center line.