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QUALITY CONTROL

QUALITY CONTROL. Rakhi Chandran Assistant Professor Department of Statistics St. Mary’s College Thrissur. BASIC DEFINITIONS. Quality refers to the good quality item which conforms to the standards specified for measurement

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QUALITY CONTROL

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  1. QUALITY CONTROL RakhiChandran Assistant Professor Department of Statistics St. Mary’s College Thrissur

  2. BASIC DEFINITIONS • Quality refers to the good quality item which conforms to the standards specified for measurement • Statistical Quality Control is a statistical method for maintaining and improving the quality of the products produced • Specifications limits are stipulated by the buyers. The buyers are expected to accept those items which fall within these limits Statistical Quality Control, RakhiChandran, St.Mary’s College

  3. VARIATIONS IN PRODUCTS • Allowable Variation –These variations are random in nature and are beyond the control of the human beings .It can be reduced but cannot be eliminated • Preventive Variation-These variations can be eliminated by taking appropriate steps Statistical Quality Control, RakhiChandran, St.Mary’s College

  4. CAUSES FOR VARIATION • Chance causes-Causes for allowable variations . • Assignable causes-Causes for preventive variations. • Variation due to chance causes lie within limits and those limits are called natural tolerance limits. Statistical Quality Control, RakhiChandran, St.Mary’s College

  5. PROCESS CONTROL AND PRODUCT CONTROL • A production process is said to be under statistical control when the manufactured items lie within the natural tolerance limits. • In this process assignable causes are absent and the process is governed by the chance causes alone. • Process control is achieved through the technique of control charts • Controlling the quality of product by critical examination at strategic points is called product control • It is achieved through sampling inspection plans Statistical Quality Control, RakhiChandran, St.Mary’s College

  6. Control Charts • It Consist of three lines. They are • Central line (C.L)-Drawn at the mean of the observations • Upper Control limit (U.C.L)-Three sigma distance above the mean • Lower Control Limit (L.C.L)-Three Sigma distance below the mean Statistical Quality Control, RakhiChandran, St.Mary’s College

  7. Types of Control Charts • Control chart for variables • Mean chart known as chart • Range chart known as R chart • Control chart for attributes • C chart • P chart • np chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  8. Chart • Compute the mean of each sample ,,….. where k denote the number of samples. • Compute = • Choose the central line as • Fix the UCL and LCL using following formula • When and are not known • a) UCL=+ • b) LCL= Statistical Quality Control, RakhiChandran, St.Mary’s College

  9. Chart (Continued) • After fixing LCL ,CL and UCL the sample mean are plotted on the chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  10. R CHART • Calculate the range R for each sample • Compute =, k being the sample number • Choose the central line as • Fix the UCL and LCL using following formula • UCL= • LCL= • After fixing LCL ,CL and UCL range values are plotted on the chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  11. C CHART • Find = • Choose the central line as • Fix the UCL and LCL using following formula • UCL=+3 • LCL= • After fixing LCL ,CL and UCL defects are plotted on the chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  12. P CHART • Compute the fraction defective P for each sample • Obtain the average fraction defectives from all the given samples. • Choose the central line as . • Fix the UCL and LCL using following formula • UCL=+3 • LCL= • After fixing LCL ,CL and UCL fraction defectives are plotted on the chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  13. np CHART • Choose the central line as n. • Fix the UCL and LCL using following formula • UCL=n+3) • LCL=n3) • After fixing LCL ,CL and UCL number of defectives are plotted on the chart Statistical Quality Control, RakhiChandran, St.Mary’s College

  14. REFERENCE • Montegomery D C , Introduction to statistical Quality Control, John Wiley And Sons Statistical Quality Control, RakhiChandran, St.Mary’s College

  15. THANK YOU Statistical Quality Control, RakhiChandran, St.Mary’s College

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