Understanding the Control Chart ComponentsThis control chart shows the median delay over time (by week), with control limits (UCL - Upper Control Limit, LCL - Lower Control Limit) and a center line (CL) indicating the process average before the intervention. Control charts like this one are used to determine whether an intervention has led to a significant change in process performance.

Analyzing the Data Before and After the Intervention

From weeks 1 to 19, before theintervention, the process appears to fluctuate around the control limits, with several points near the upper control limit, indicating higher variation and a generally stable process around a higher median delay.

After week 20, following the intervention, the data points show a clear shift downward, consistently staying below the previous control line (CL). The process median delay has notably reduced, and all data points fall within a new, lower range.

Conclusion Based on the Control Chart

The consistent reduction in median delay and the clustering of data points below the previous center line indicate a shift in performance rather than mere random variation. This type of change, sustained over multiple weeks, strongly suggests that the intervention had a substantial impact on reducing the delay.

While there may also be a decrease in variation, the primary visible effect is a shift in performance toward lower median delay values.

Rationale for Selecting Answer BThe correct answer is B. The intervention resulted in a shift in performance, as the chart shows a distinct change in the process level post-intervention, indicating an improvement.

NAHQ 'Quality Improvement in Healthcare: Statistical Process Control'

'Interpreting Control Charts forProcess Improvement' (NAHQ, 2020)