ITPEC FE Morning October 2020 Question 54
Which of the following is used in quality management to help determine whether a process is stable by using plotted time-series data and limit lines?
Explanation
Control chart — it plots a process measure in time order between control limits to show whether the process is stable.
A control chart plots measurements over time around a centre line, with an upper control limit (UCL) and a lower control limit (LCL), usually three standard deviations either side. Points outside the limits, or non-random patterns such as a long run on one side of the centre line, signal that the process is out of control. Points scattered at random inside the limits indicate a stable process.
Answer (c)
Why not others:
- •(a) Brainstorming — a group technique for generating ideas freely; no data are plotted
- •(b) Cause-and-effect diagram — the fishbone (Ishikawa) diagram arranges the possible causes of a problem; it has no time axis and no limits
- •(d) Run chart — plots data in time order to show trends, but has no control limits, so it cannot tell whether a variation goes beyond what the process normally produces
Key rule: Time-series data with limit lines is a control chart; the same plot without limits is a run chart.
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