X-bar and S control charts of LOS X-bar and S control charts of LOS. A, Statistical process control chart (X-bar) for ALOS of pediatric patients with asthma where each dot represents the ALOS for the corresponding month. X-bar and S control charts of LOS. A, Statistical process control chart (X-bar) for ALOS of pediatric patients with asthma where each dot represents the ALOS for the corresponding month. B, SD control chart for ALOS of pediatric patients with asthma. Kathleen W. Bartlett et al. Hospital Pediatrics 2017;7:31-38 ©2017 by American Academy of Pediatrics