Quality Control McGraw-Hill/Irwin Copyright © 2012 by The McGraw-Hill Companies, Inc All rights reserved You should be able to: List and briefly explain the elements in the control process Explain how control charts are used to monitor a process, and the concepts that underlie their use Use and interpret control charts Perform run tests to check for nonrandomness in process output Assess process capability Student Slides 10-2 Quality Control A process that evaluates output relative to a standard and takes corrective action when output doesn’t meet standards If results are acceptable no further action is required Unacceptable results call for correction action Student Slides 10-3 Inspection An appraisal activity that compares goods or services to a standard Inspection issues: How much to inspect and how often At what points in the process to inspect Whether to inspect in a centralized or on-site location Whether to inspect attributes or variables Student Slides 10-4 Quality control seeks Quality of Conformance A product or service conforms to specifications A tool used to help in this process: SPC Statistical evaluation of the output of a process Helps us to decide if a process is “in control” or if corrective action is needed Student Slides 10-5 Control Chart A time ordered plot of representative sample statistics obtained from an ongoing process (e.g sample means), used to distinguish between random and nonrandom variability Control limits The dividing lines between random and nonrandom deviations from the mean of the distribution Upper and lower control limits define the range of acceptable variation Student Slides 10-6 Variables generate data that are measured Mean control charts Used to monitor the central tendency of a process “x- bar” charts Range control charts Used to monitor the process dispersion R charts Student Slides 10-7 k k x= ∑ xi i =1 k where x = Average of sample means x i = mean of sample i R= ∑R i =1 i k where R = Average of sample ranges Ri = Range of sample i k = number of samples Student Slides 10-8 Used to monitor the central tendency of a process x − chart Control Limits UCLx = x + A2 R LCLx = x − A2 R where A2 = a control chart factor based on sample size, n Student Slides 10-9 Used to monitor process dispersion R Chart Control Limits UCLR = D4 R LCLR = D3 R where D3 = a control chart factor based on sample size, n D4 = a control chart factor based on sample size, n Student Slides 10-10 Student Slides 10-11 Attributes generate data that are counted p-Chart Control chart used to monitor the proportion of defectives in a process c-Chart Control chart used to monitor the number of defects per unit Student Slides 10-12 Once a process has been determined to be stable, it is necessary to determine if the process is capable of producing output that is within an acceptable range Tolerances or specifications Range of acceptable values established by engineering design or customer requirements Process variability Natural or inherent variability in a process Process capability The inherent variability of process output (process width) relative to the variation allowed by the design specification (specification width) Student Slides 10-13 Student Slides 10-14 Quality is a primary consideration for nearly all customers Achieving and maintaining quality standards is of strategic importance to all business organizations Product and service design Increase capability in order to move from extensive use of control charts and inspection to achieve desired quality outcomes Student Slides 10-15 ... briefly explain the elements in the control process Explain how control charts are used to monitor a process, and the concepts that underlie their use Use and interpret control charts Perform run tests... or on-site location Whether to inspect attributes or variables Student Slides 10-4 Quality control seeks Quality of Conformance A product or service conforms to specifications A tool used... process x − chart Control Limits UCLx = x + A2 R LCLx = x − A2 R where A2 = a control chart factor based on sample size, n Student Slides 10-9 Used to monitor process dispersion R Chart Control Limits