Figure 1
The illustration displays two side-by-side control charts labeled “(a)” on the left and “(b)” on the right, both enclosed in rectangular frames. Chart (a) shows a legend at the bottom showing two line types: a blue line labeled “C U S U M” and a red dashed line labeled “5 percent Significance.” The vertical axis ranges from negative 10.0 to 10.0 with an interval of 2.5, and the horizontal axis spans from 12 to 22 with an interval of 1. A horizontal line is drawn from the vertical axis at value 0. The blue curve starts from (12, 0) and decreases till (13.971, negative 1.601) and then increases through (18.057, 3.594) and ends at (22, 5.231). The top red dashed line starts from (12, 3.238) and increases linearly, ending at (22, 9.573). The bottom red dashed line starts from (12, negative 3.025) and decreases linearly, ending at (22, negative 9.359). Chart (b) shows a legend at the bottom showing two line types: a blue line labeled “C U S U M of squares” and a red dashed line labeled “5 percent Significance.” The vertical axis in this chart ranges from negative 0.4 to 1.6 with an interval of 0.4, and the horizontal axis spans from 12 to 22 with an interval of 1. A horizontal line is drawn from the vertical axis at value 0. The blue curve starts from (12, 0) and then increases in a step-like pattern, which passes through (16.977, 0.387) and ends at (22, 1.018). The top red dashed line starts from (12, 0.451) and increases linearly, ending at (22, 1.359). The bottom red dashed line starts from (12, negative 0.244) and increases linearly, ending at (22, 0.664). Note: All numerical data values are approximated.Plot of CUSUM (a) and CUSUMQ (b) for the Omani tax revenue model. Source: Estimation’s output