Royal North Shore Hospital, a 650-bed teaching hospital in Sidney, Australia, uses control charts for quality management. One control chart used shows tire number of outpatients waiting more than 30 days for elective surgery' (see below). Because the number was increasing (see the plot to the left of X up to 3 months before A), the hospital appointed a waiting list coordinator (3 months before X) who helped reduce the number (see the plot between 3 months before A and Y), and Later reduced further by reducing the holidays for elective surgery from six weeks to three weeks during Christmas (see tire plot to the right of T). The control limit lines are 3 standard deviations of the values during each of the 3 sub periods from their centre lines. Determine what type of control chart this is.