Page 215 - DMGT524_TOTAL_QUALITY_MANAGEMENT

Total Quality Management

Notes Elements of a Control Chart: A control chart consists of:
A central line,
An upper control limit,

A lower control limit, and
Process values plotted on the chart.

14.4.1 Steps for Developing Control Charts

Steps required for developing and using control charts are as follows:
1. Preparation

Choose the variable or attribute to be measured
Determine the basis, size and frequency of sampling
Set up the control charts
2. Data collection
Record the data

Calculate relevant statistics: averages, ranges, proportions, and so on
Plot the statistics on the chart
3. Determination of trial control limit

Draw the centre line (process average) on the chart
Compute the upper and lower control limits
4. Analysis and interpretation
Investigate the chart for lack of control
Eliminate out-of-control points

Recompute control limits if necessary
5. Use as a problem-solving tool
Continue data collection and plotting

Identify out-of-control situations and take corrective action
6. The chart may contain other optional features
Upper and lower warning limits, drawn as separate lines, typically two standard
deviations above and below the centre line

Division into zones, with the addition of rules governing frequencies of observations
in each zone

Annotation with events of interest, as determined by the Quality Engineer in charge
of the process’s quality
However in the early stages of use the inclusion of these items may confuse inexperienced chart
interpreters.

In general, the chart contains a Centreline that represents the mean value for their-control
process. Two other horizontal lines, called the Upper Control Limit (UCL) and the Lower Control

210 LOVELY PROFESSIONAL UNIVERSITY