BUS310_L11_2.15.11

# BUS310_L11_2.15.11 - Process Control Charts Lecture 11...

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Lecture 11: Process Control Charts
Example: Possible Process Improvement Project for a Hotel Goal: Increase customer satisfaction by improving: Readiness of the rooms (categorical), Delivery time for luggage (numerical) Are proportions of ready rooms acceptable? Are delivery times acceptable? Are they consistent on a daily basis? Are inconsistencies due to special events or flaws in the processes of Process Control Charts (Chapter 14)

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Control Charts : Common statistical tools to monitor and control processes Collecting data over time to improve a process Why is there variation in the process ? Special versus Common cause Control limits Rules for determining when a process is out of control 1. A point falls outside the 3 standard deviation control limits 2. Trend pattern: Eight consecutive points above the center line (or 8 below) 3. Trend pattern: Eight increasing or eight decreasing points Chapter14, Annotated Excel files Lecture Outline
1. Collect data sequentially over time Collect samples (subgroups) from the output of a process over time Room readiness (yes/no) and luggage delivery time (in minutes) 2. Calculate sample statistic for each subgroup Categorical variables (rooms ready or not) : sample proportion (rooms not ready) P-Chart Numerical variables: range, mean (delivery times) Range and Mean charts 3. Plot the values over time (Time Series (Lecture 3)) 4. Add control limits Purpose: Visual Display of the data representing the process Construct Control Chart

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Special Cause of Variation (Outlier): A measurement this far from the process mean is very unlikely if only expected variation is present Process Mean UCL LCL +3σ - time Common Cause Variation: range of expected variability Special Cause of Variation (Outlier) Control chart shows if variation in data are due to: Common Cause Variation Naturally occurring and expected fluctuations (normal variation in materials, tools, machines, operators, and the environment) Reduced only by changing the process Special Cause Variation Fluctuations or patterns not inherent to a process, often unusual events Problems to be corrected or opportunities Data outside control limits or trends (Rules 1, 2, 3)
Using Control Charts H0: The process is in control i.e., variation is only due to common causes H1: The process is out of control i.e., special cause variation exists (beside common causes) If the process is out of control, steps should be taken to find and eliminate the special causes of variation

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Each dot corresponds to the sample statistic for a subgroup (one of the samples collected of the process output over time) Categorical variables (rooms ready or not) : sample proportion (rooms not ready) P-Chart Numerical variables: range, mean (delivery times) Range and Mean charts Process In Control Process in control: points are randomly distributed around the center line and all points are within the control limits
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## This note was uploaded on 04/07/2011 for the course BUS 311 taught by Professor Reardon,j during the Spring '08 term at University of Hawaii, Manoa.

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BUS310_L11_2.15.11 - Process Control Charts Lecture 11...

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