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Unit 6: Quality Assurance and Control

Notes
B
C C G g
b
P(b) =
C N n
where,
P(b) = Probability of finding ‘b’ number of bad items in the sample of size ‘n’ taken from the lot
of size N

B = Number of bad items in the lot;
G = Number of good items in the lot;

g = Number of good items in the sample;
b = Number of bad items in the sample;
P(a) = Probability of Acceptance of the lot.

Figure 6.2: Operating Characteristic Curve

6.11 Importance or Benefits of SQC

The technique of SQC has become very popular since the days of World War II. In modern
Industry it has become a necessity as it offers the following benefits:

1. It saves on rejection: In the absence of SQC technique, many products may be found
defective and worthless at the of manufacturing process and have to be thrown away as a
scrap. SQC helps to avoid such a situation and thus saves the cost of labour & material
involved in the production of defective items.

SQC technique measures the extent of defect and certain defective products may be
approved with reworking to the level of acceptable standards. It helps in deciding whether
to do reworking or not and hence helps in reduction of loses due to unnecessary working.

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