OM 300 1st Edition Lecture 11 Outline of Last Lecture I Chart Examples II Inspection III Attributes v Variables IV Service Quality Outline of Current Lecture I Purpose of a Control Chart II Control Charts Current Lecture Darden Video Quality Farm to plate Inspect all food Use TQM tools to ensure quality Train workers from global supply chain The objective of a process control system is to provide a statistical signal when assignable causes of variation are present If you can t describe and measure it then you can t control or improve it Illustrations WWII Defense Industries o Sampling procedures to increase productivity and quality Englehard catalytic cores o Percentage of defective cores Milliken industrial fabrics o Number of defects per 100 yards Thermalex thermal tubing o Heigh x4 and width variables Land s End customer service order fulfillment o Percentage of correctly filled orders o Number of errors per line item Hospital pharmacy o Prescription error rate Statistical Process Control SPC Variability is inherent in every process These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Natural or common causes o Assignable or special causes Provides a statistical signal when assignable causes are present Detect and eliminate assignable causes or variation Natural Variations Also called common causes Affect virtually all production processes Expected amount of variation Output measures follow a probability distribution For any distribution there is a measure of central tendency and dispersion If the distribution of outputs falls within acceptable limits the process is said to be in control Assignable Variations Also called special causes of variation o Generally this is some change in the process Variations that can be traced to a specific reason The objective is to discover when assignable causes are present o Eliminate the bad causes o Incorporate the good causes Sampling Measure or inspect only a portion of the products or transactions at regular periodic time intervals Reduces inspection time Reduces the opportunity for bad quality products or services to reach the customer Good strategy when the process is well designed and incontrol Sample size and sampling frequency are important decisions that impact the effectiveness of the SPC procedure Control Charts constructed from historical data the purpose is to help distinguish between natural variations and variations due to assignable causes Types of Quality Data Attribute data Product characteristic evaluated with a discrete choice Good Bad Yes No Variable Data Product characteristic that can be measured Length size weight height time velocity Central Limit Theorem Regardless of the distribution of the population the distribution of sample means drawn from the population will tend to follow a normal curve I II The mean of the sampling distribution will be the same as the population mean The standard deviation of the sampling distribution will equal the population standard deviation divided by the square root of the sample size n Control Charts for Variables Characteristics that can take any real value May be in whole or in fractional numbers Continuous random variables o x chart tracks changes in the central tendency o R chart indicates a gain or loss dispersion
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