Question

Before control charts are invented, the process engineers have been routinely making line charts that illustrate the relationship between the quality characteristic measurements and the measurement index. These line charts show how the quality characteristics vary over time. Describe at least two differences between control charts and these line charts and discuss what are the benefit of using control charts over using line charts.

Answer 1

Control Charts:

• When controlling ongoing processes by finding and correcting problems as they occur.
• When predicting the expected range of outcomes from a process.
• When determining whether a process is stable (in statistical control).
• A control chart always has a central line for the average, an upper line for the upper control limit and a lower line for the lower control limit.

Line Charts:

• Run charts cannot tell you if a process is stable.
• When controlling ongoing processes by finding and correcting problems is not possible.
• Run Chart dosen't have an upper control limit , lower control limit and central line.