Page 224 - DMGT524_TOTAL_QUALITY_MANAGEMENT

Unit 14: Statistical Process Control

(ii) p chart Notes
CL = p = 0.06.
p
( p1 p ) 0.06 × 0.94
−
−
LCL = p − 3 = 0.06 3 = 0.0096
p
n 200
( p1 p ) 0.06 × 0.94
−
+
UCLp = p + 3 = 0.06 3 = 0.110
n 200
Illustration 5: Draw a suitable control chart for the following data pertaining to the number of
foreign coloured threads (considered as defects) in 15 pieces of cloth of 2m × 2m in a certain
make of synthetic fibre and state your conclusions.
7, 12, 3, 20, 21, 5, 4, 3, 10, 8, 0, 9, 6, 7, 20.

Solution: The required values for considering a chart are as follows:
(i) The number of defects in each of 15 pieces of cloth are:
C: 7, 12, 3, 20, 21, 5, 4, 3, 10, 8, 0, 9, 6, 7, 20

Average number of defects ( C )

Σ C 135
C = = = 9 .
n 15
3s control limits for C chart are given by:

9
UCL = C + 3 C = + 9 18 .
=
C
0
9
9
LCL = C − 3 C = − = .
C
CL = C = 9
C
Task The number of customer’s complaints received daily by an organisation are as
follows:

Day : 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
No. of customer complaints : 2 3 0 1 9 2 0 0 4 2 0 7 0 2 4

Does it mean that the number of complaints is under statistical control? Establish the
control scheme for the future.

Self Assessment

Fill in the blanks:

9. The control chart is the fundamental tool of statistical process control, as it indicates the
range of …………………… that is built into a system
10. Control charts are ……………………-making tools.

11. Control …………………… are lines on the charts that represent the current acceptable
level of variation in the process.

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