Page 169 - DMGT501_OPERATIONS_MANAGEMENT

Unit 6: Quality Assurance and Control

(c) Read value of A from the statistical table Notes
2
(d) Value of Control Limits for the average are given by
X + or – A R
2
(e) Plot the average on the graph and study the graph carefully for any systematic or
other variations of the limits and investigate causes thereof.

Control Charts in Process Control

A Control Chart is the graphical representation between the order of sampling along x-axis and
statistics (functions of the observed values of the Variable) along y-axis.
The Central Line (CL) displays the standard line, and UCL and LCL display the Upper Control
Limits and Lower Control Limits. These Control Limits (usually 11.7% of the values arising
from Chance causes) are used to distinguish between the Chance causes and the Assignable
causes of variation. The control charts are useful for operators and hence should be displayed at
convenient positions.

Maintenance and Usage of Control Charts

The samples are taken at regular suitable intervals and statistic plotted on a chart. If the point
remains within the ‘UCL and LCL’, the process is allowed to continue. The product so produced
is called a good lot.
If there is evidence of lack of control, the process should be stopped, investigated, corrected and
restarted. Till the process gets stabilised, keep these goods separately segregated for good and
bad separately.
A point outside the control limit is an index of out of control situation whereas the pattern of
points indicate the nature of action desired at any point of time.

Types of Control Charts

There are many types of Control Charts suited to various types of situations. The major ones are
briefly described below:
1. (X, R ) charts:
(a) (X, R) charts are applicable to variable type of data.
(b) These charts are used to control the individual characteristic (X, R).

(c) These charts provide the maximum information from the available data on ‘Mean’
and ‘Variation’ for the control.
(d) Small samples will suffice

Statistic Standard Control Limits
UCL LCL

X X X + A R X – A R
2
2
R R D R D R
4 3
where A , D , D are read from Statistical table against the selected sample size.
2 4 3

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