Page 114 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 5: Quality Management

Self Assessment Notes

Fill in the blanks:
14. The philosophy underlying Six Sigma is to reduce ……………… variation.
15. Six Sigma refers to a disciplined, ……………… approach and methodology for eliminating
defects in any process.

16. For a Six Sigma process with only one specification limit, there are ……………… process
standard deviations between the mean of the process and the customer's specification
limit.
5.8 Acceptance Sampling

The approach to control production quality level differs somewhat with the type of production
involved.

 In jobbing production with its low volume of output to particular technical specifications,
emphasis is placed on deciding the appropriate inputs to achieve the desired specifications
and setting up the operation correctly and on 100 per cent inspection.

 In high volume production, inspecting the critical characteristics on every item, i.e., 100
per cent inspection, is not very practical. Since uniformity is not possible, the key question
is, "How much variation exists in the processes?" The answer begins with the process of
selecting representative units of output, called samples. Samples are measured for the key
product characteristics, in order to ensure that the specification limits are not exceeded.
This is called acceptance sampling and has become a norm in most of these industries.

Sampling by Attribute

When the inspection has to basically decide whether or not the item is within the specified
limits, the procedure is called 'sampling by attributes'.

 Attribute sampling merely calls for making a yes-no decision.
 This is done on manufactured parts by means of gauges. These gauges need not be read,
they only examine if the critical dimensions are within limits.
 The advantage of attribute counts is that less effort and fewer resources are needed than
for measuring variables.

 The disadvantage is that even through attribute counts can reveal that quality of
performance has changed it may not be of much use in indicating by how much.
 Attribute sampling can be applied to 'services' also.

Example: A count may be used to determine the proportion of airline flights arriving
within 15 minutes of their scheduled times. The count may show that the proportion of airline
flights arriving within 15 minutes has declined, but the result may not show how much beyond
their scheduled times has decline. The result will not show how much beyond the 15 minute
allowance the flights are arriving. For that, the actual deviation from the scheduled arrival, a
variable would have to be measured.

Sampling by Variables

When the inspection calls for measuring the critical characteristics on each sample and recording
the measurements, it is called 'sampling by variables'.

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