STATSTICAL TECHNIQUES FOR QUALITY CONTROL — DU B.Sc (hons) Statistics Previous Year Question Papers

2. Attempt Six questions in all. 4. Attempt two questions from section A and three question from Section B. 6. Required Statistical tables are attached with the paper. 1. (a) Explain rational sub-grouping and its role in control chart analysis. (7) Explain the difference between random causes and assignable causes. What part do they play in the operation and interpretation of Shewhart control chart? (b) Define Quality. Discuss its concept, application, and importance in manufacturing processes. Introduce ISO quality standards and discuss their significance in establishing quality management systems. 2. (a) What are modified control limits? Explain how they are derived. Why it is required to construct modified control limits? (7) (b) Discuss and derive the construction of X and R control charts for controlling process average and process variability, when population parameters are both (i) known and (b) 25 Samples of size n = 5 each are taken from a manufacturing process every hour. Assuming quality characteristic is normally distributed, X and Rare computed for each sample. Given, (i Find the 3o control limits for the & charts. (i) If the specifications limits are 26.40 ‡ 0.5 0, then estimate the fraction non-conforming, assuming that both charts exhibit control. (8) 4. (a) Explain sampling inspection plans for attributes. Define the OC and ASN function. (7) (b) Obtain the control limits for number of defects per unit; also obtain the control limits for average number of defects per unit giving clearly the statistical • concept used? (8) 5. (a) What is an OC curve? Draw its important points on the plot. Obtain its expression in a single sampling plan. (7) (b) A process is being controlled with a fraction nonconforming control chart. The process average has been shown to be 0.07. Three-sigma control limits are used, and the procedure calls for taking daily samples of 400 items. (i) Calculate the upper and lower control limits. (ii) If the process average should suddenly shift to 0.10, what is the probability that the shift would be detected on the first subsequent sample? (8) 6. (a) Explain briefly the Process and Product control, detailing the roles of both in ensuring quality in manufacturing process. (7) (b) Define AOQ and AOQL. For a single sampling plan obtain an expression for 7. (a) Distinguish among natural tolerance limits, control limits and specification limits. Discuss the ideal situation? (7) (b) A lot consists of 1000 items, of which 100 are defective. Using a single sampling plan with a sample size of 50 and an acceptance number of 3, calculate the probability of accepting the lot. 8. (a) Define Statistical Process control (SPC). Explain in brief the tools of Statistical Process control (SPC). (7)