* Learning Objectives:
1. List and briefly explain the elements in the control process 2. Explain how control charts are used to monitor a process, and the concepts that underlie their use 3. Use and interpret control charts
4. Perform run tests to check for nonrandomness in process output 5. Assess process capability
* What is Quality Control?
6. 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
* Phases of Quality Assurance:
* An appraisal activity that compares goods or services to a standard * Inspection issues:
1. How much to inspect and how often
2. At what points in the process to inspect
3. Whether to inspect in a centralized or on-site location 4. Whether to inspect attributes or variables
* How Much to Inspect?
* Typical Inspection Points:
* Raw materials and purchased parts
* Finished products
* Before a costly operation
* Before an irreversible process
* Before a covering process
* Effects on cost and level of disruption are a major issue in selecting centralized vs. on-site inspection * Centralized
* Specialized tests that may best be completed in a lab * More specialized testing equipment
* More favorable testing environment
* Quicker decisions are rendered
* Avoid introduction of extraneous factors
* Quality at the source
* Statistical Process Control (SPC)
* 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 * Process Variability
* Two basic questions: concerning variability:
* Issue of Process Control
* Are the variations random? If nonrandom variation is present, the process is said to be unstable. * Issue of Process Capability
* Given a stable process, is the inherent variability of the process within a range that conforms to performance criteria? * Variation
* Random (common cause) variation:
* Natural variation in the output of a process, created by countless minor factors * Assignable (special cause) variation:
* A variation whose cause can be identified.
* A nonrandom variation
* Sampling and Sampling Distribution
* SPC involves periodically taking samples of process output and computing sample statistics: * Sample means
* The number of occurrences of some outcome
* Sample statistics are used to judge the randomness of process variation * Sampling Distribution
* A theoretical distribution that describes the random variability of sample statistics * The normal distribution is commonly used for this purpose * Central Limit Theorem
* The distribution of sample averages tends to be normal regardless of the shape of the process distribution
* Control Process
* Sampling and corrective action are only a part of the control process * Steps required for effective control:
* Define: What is to be controlled?
* Measure: How will measurement be accomplished?
* Compare: There must be a standard of comparison
* Evaluate: Establish a definition of out of control * Correct: Uncover the cause of nonrandom variability and fix it *...