Page 240 - DMGT524_TOTAL_QUALITY_MANAGEMENT

Unit 14: Statistical Process Control

point an investigation is conducted to discover unusual and abnormal conditions (e.g. Notes
tool breakdown, use of wrong raw material, temperature controller failure, etc.).
Subsequently, corrective actions are taken to remove the abnormality.

In addition to the use of control charts. SPC also monitors process capability an indicator
of the adequacy of the manufacturing process to meet customer requirements under routine
operating conditions.

The control chart is the fundamental tool of statistical process control, as it indicates the
range of variability that is built into a system (known as common cause variation). Thus,
it helps determine whether or not a process is operating consistently or if a special cause
has occurred to change the process mean or variance.
Control charts are decision-making tools. They provide an economic basis for deciding
whether to alter a process or leave it alone. Control charts are problem-solving tools and
provide a basis to formulate improvements actions.
Control charts help us learn more about process variation. They determine whether a
process is in a state of statistical control or out-of-control. They are used to estimate the
process parameters (mean, variation) and assess the performance of a process or its
capability.
Thus, SPC aims at maintaining a stable, capable and predictable process.

14.8 Keywords

Control Chart: Control chart is the fundamental tool of statistical process control, as it indicates
the range of variability that is built into a system (known as common cause variation).
Control Limits: Control limits are lines on the charts that represent the current acceptable level
of variation in the process.
Cycle: Short and repeated patterns in the chart with alternating high peaks and how valleys
called cycles are the result of cause that come and go on a regular basis.

Design of Experiments: Application of statistical methods for producing high quality, robust
products and process designs.
Process Capability: The range over which the ‘natural variation’ of a process occurs and is
determined by the system of common/random causes.
Statistical Process Control (SPC): A process to control the variability of output using control
charts.
Statistical Quality Control (SQC): Use of statistical methods to improve or enhance quality for
customer satisfaction. It involves monitoring a process to identify the unique causes of variation
for signalling appropriate corrective actions.
Trends: A trend is the result of some cause that gradually affects the quality characteristics of the
product and causes the point on a control chart to gradually move up or down from the centre
line.

14.9 Review Questions

1. What do you mean by statistical process control?

2. Describe the advantages of statistical process control.
3. Describe control limits are not an indication of capability.

LOVELY PROFESSIONAL UNIVERSITY 235