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OM 300
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David Cooper
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Lecture 6

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Operations Management

OM 300

David Cooper

Summer

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Operations Management Chapter 6 Supplement Notes
Statistical Process Control
Statistical Process Control
• The objective of a process control system is to provide a _______ when
assignable causes of variation are present
➢ “If you can’t describe and _______ it, then you can’t control or ____ it?
Illustrations
• WWII Defense Industries
➢ Sampling procedures to increase productivity and quality
• Engelhard – catalytic cores
➢ Percentage of defective cores
• Milliken – industrial fabrics
➢ Number of defects per 100 yards
• Thermalex – thermal tubing
➢ Height (x4) and width (variables)
• Land’s End – customer service, order fulfillment
➢ Percentage of correctly filled orders
➢ Number of errors per line item
• Hospital pharmacy
➢ Prescription error rate
Statistical Process Control (SPC)
• _________ is inherent in every process
➢ _______ or common causes
➢ Special or _______ causes
• Provides a statistical signal when assignable causes are present
• _______ and eliminate assignable causes of variation
Natural Variations
• Also called ______ causes
• Affect virtually all production processes
• Expected amount of variation
• Output measures follow a _______ 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 “____ _______”
Assignable Variations
• Also called special causes for variation
➢ Generally this is some ______ in the process
• Variations that can be traced to a specific reason
• The objective is to ________ when assignable causes are present ➢ Eliminate the bad causes
➢ 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 “in-control”
• Sample size and sampling frequency are important decisions that impact the
effectiveness of the SPC procedure
Control Charts
• Constructed from historical data, the purpose of control charts 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 them to follow a normal curve
➢ The mean of the sampling distribution will be the same as the population
mean
o X = mean
➢ The standard deviation of the sampling distribution will equal the
population standard deviation divided by the square root of the sample
size, n
o
Control Charts for Variables
• Characteristics that can take any real value
• May be in whole or fractional numbers
• Continuous random variables
• X-chart tracks changes in the ______ tendency
• R-chart indicates a gain or loss in ________
R-Chart • Type of _____ control chart
• Shows sample ranges over time
➢ _______ between smallest and largest values in sample
• Monitors process variability
• Independent from process mean
Steps In Creating Control Charts
1. Take samples from the population and computer the appropriate sample
statistic
2. Use the sample statistic to calculate control limits and draw the control chart
3. Plot sample results on the control chart and determine the state of the
process (in or out of control)
4. Investigate possible assignable causes and take any indicated actions
5. Continue sampling from the process and reset the control limits when
necessary
Control Charts for Attributes
• For variables that are ______
➢ Defective/nondefective, good/bad, yes/no, acceptable/unacceptable
• Measurement is typi

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