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Quality Control
Prof. Dr. Alagar Rangan
Quality Control
• Quality is conformance to specifications
• Quality characteristic
– Variable
– Attribute
• Variable: Quality characteristic that can be measured on a numerical
scale length, diameter, density, etc.
• Attribute: That cannot be measured on a numerical scale, Yes, No; Good,
Ok, Bad; etc.
• Quality Control: A system that is used to maintain designed Quality in a
product or service.
• Father of Quality Control: Edward Deming.
Quality Control
• comprehensive approach to improving Quality is TQM (Total Quality
• Graphical Methods: Histograms, Cause and effect Diagrams, etc.
Quality Control
• Control charts: CL, UCL, and LCL.
• Average and Variations.
• Average – Mean.
• Variation – Range.
Quality Control
• Control Charts based on Normal distribution.
• Normal distribution is symmetric.
• In a production process, measurements of a Quality are normally
Quality Control
• Reliability
T: Failure time, a random variable
Reliability R(t) is the probability that a system is working at time t.
R(t) = P(T>t)
Quality Control
• Series system
A series system works if all the components work.
• Parallel system
A parallel system works if at least one of the components works.
Example 1. Quality Control
Lathe Machining Car Parts:
• Quality Characteristic: Thickness
Variable Characteristic
Process Mean Thickness
Process Standard Deviation of Thickness
Known or Estimated
Company wishes to control the thickness of
Take samples of size 5 parts every hour
Calculate sample mean (x1+x2+...+x5)/5
These means are normally distributed
Using this idea, draw control lines
Upper and lower control line
If every value of sample mean falls within UCL
& LCL then process is under control
• If not, stop production
• Check for assignable causes
• Then restart production
• Improves quality, saves cost, reduces lead
Example 2. Reliability
• Failure Time of a Solid State Power Unit
• Failure times are random variables
• Reliability : The power unit operates
continuously for a fixed period without failure
• Expected time to failure
• Design Life: How to design power units to
have a required reliability
• Rate of failure of systems
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