Fig 3.

Tracking of time to receipt of COVID-19 RT-PCR test results by date of test and change by testing site. For the X-bar portion of the X-bar and S control chart, each data point represents the average turnaround time of all tests collected that day. The center line is the average of the daily data points, subgrouped by preintervention and following system changes, when special cause is detected according to Western Electric methodology. Control limits on the X-bar are calculated, on the basis of average SD of the subgrouped points, multiplied by a factor for group size for each data point.³⁰