Introduction to Control Charts Supplement to “Using Control Charts in a Healthcare Setting” case 2 Variation Variety may be the “spice of life”, but in the context of product quality, process performance, & service delivery, variation is a BAD thing Bears Fans Cheer! Bears Fans Groan! Career 187 made 218 attempts 85.8% success rate Robbie Gould Image - http://www.nflfootballpicks.org/authors/6/Vic-Busey?Page=9 Robbie Gould Stats - http://www.nfl.com/player/robbiegould/2506264/careerstats Goal Post Image - http://www.partycheap.com/Jointed_Goal_Post_p/50474.htm Bears Fans Groan! Customer Specification Limits 3 All Processes are Variable Special Cause Variation • • Unpredictable Intermittent Good Expected Variation Change Common Cause Variation • • Inherent Random New Expected Variation But the key is… “We must understand variation.” W. Edwards Deming 4 Variation & Customer Specifications Expected Variation Customer Specs Expected Variation Customer Specs 5 Reacting to Variation One day, Tiger went to the driving range & hit the same way each time. Not satisfied with the results, he tried again the next day, but tried to improve each shot based upon the last one. What went wrong? Tiger Woods Image #1 - http://moodfresher.blogspot.com/2012/02/tiger-woods-wallpapers.html Tiger Woods Image #2 - http://www.topnews.in/light/people/tiger-woods 6 Over-Reaction to Common Cause Variation? Examples An operator reacting in the opposite direction based on the previous result Reacting to a single customer complaint without understanding if it is common to many customers • A manager reacting to a single data point Setting the current period’s sales quota based on last periods overage or underage Setting the next period’s budget as a percentage of the last period’s budget The stock market reacting to good news or bad news Unintended Consequences Existing workers training the new workers • Symptom of no Standard Work (Lean concept) Over-reacting to Common Cause Variation leads to MORE variation, not less! 7 Understanding Variation Descriptive Statistics Average Percentage Median Range Visual Tools Histograms • An approximation of the distribution’s shape Box Plots Scatter Diagrams Run Charts Control Charts • SPC – Statistical Process Control 8 Run Charts Displays observed data in a time sequence So what? 3 different run charts with the same distribution 9 Control Charts Run Charts w/Control limits Typically ± 3 standard deviations “Rules” for flagging special causes Customer Specs Expected Variation Customer Specs Control Limits & Specification Limits are NOT the same 10 Some Control Chart Basics Typically, data is plotted as it happens However, a control chart can be created in an retrospective manner This approach requires some judgment when determining if there has been a process shift There are standard control chart tests (see next page) If one of these tests (or conditions) are violated, it is flagged which is supposed to prompt an investigation as to “why” it happened 11 Control Chart Tests Attribute Tests Numbers of the Line = Failed Test Test 1 2 3 4 5 Plus More Tests for Variable Data 6 7 8 12 Control Chart Selection Guide Source: Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2005. 13 Control Chart Selection Guide (another view) Variable Control Charts Chart Type Characteristic Monitored Individual X Individual Values Location Xbar Sample Mean Spread R MR D Sample Range Moving Range Sample Standard Deviation Attribute Control Charts Chart Type p np u c Characteristic Monitored for defectives (%) – sample size varies for defectives (%) – sample size fixed for defects – sample size fixed for defects – sample size varies 14 More Control Chart Basics What they can do… Identify a process that is out-of-control Provide statistical evidence when a process has actually changed (either for better or for worse) What they cannot do… Identify why the process is out-of-control • That is left for you to investigate 15 Never Stop Asking “Why?” Image - http://1.bp.blogspot.com/_IbpK-GhiWSc/TJTfX63zpVI/AAAAAAAACOI/na3ZgyUFd2M/s1600/1227573326bxGKsyD.jpg 16