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Introduction to Control Charts

Supplement to “Using Control Charts in a Healthcare Setting” case

Variation

Variety may be the “spice of life”, but in the context of product quality, process performance, & service delivery, variation is a BAD thing   

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Robbie Gould Image - http://www.nflfootballpicks.org/authors/6/Vic-Busey?Page=9Robbie Gould Stats - http://www.nfl.com/player/robbiegould/2506264/careerstatsGoal Post Image - http://www.partycheap.com/Jointed_Goal_Post_p/50474.htm

Career187 made218 attempts85.8% success rate

CustomerSpecification

Limits

Bears Fans Cheer!

Bears Fans Groan!

Bears Fans Groan!

All Processes are Variable

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“We must understand variation.”W. Edwards Deming

But the key is…

Common Cause Variation• Inherent• Random

Special Cause Variation• Unpredictable• Intermittent

Change

Expected Variation

NewExpected Variation

Good

Variation & Customer Specifications

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Customer Specs

Expected Variation

Customer Specs

Expected Variation

Reacting to Variation

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Tiger Woods Image #1 - http://moodfresher.blogspot.com/2012/02/tiger-woods-wallpapers.htmlTiger Woods Image #2 - http://www.topnews.in/light/people/tiger-woods

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?

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!

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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

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Run Charts

Displays observed data in a time sequence So what?

3 different run charts with the same distribution

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Control Charts

Run Charts w/Control limits Typically ± 3 standard deviations “Rules” for flagging special causes

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Customer Specs

Expected Variation

Customer Specs

Control Limits & Specification Limits are NOT the same

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

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Control Chart Tests

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Attribute

Tests

PlusMore Tests

for

VariableData

Test1

2

3

4

5

6

7

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Numbers of the Line= Failed Test

Control Chart Selection Guide

13Source: Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2005.

Control Chart Selection Guide (another view)

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Chart Type Characteristic MonitoredIndividual X Individual Values

Xbar Sample Mean

R Sample RangeMR Moving RangeD Sample Standard Deviation

Location

Spread

Variable Control Charts

Chart Type Characteristic Monitoredp for defectives (%) – sample size variesnp for defectives (%) – sample size fixedu for defects – sample size fixedc for defects – sample size varies

Attribute Control Charts

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

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Never Stop Asking “Why?”16

Image - http://1.bp.blogspot.com/_IbpK-GhiWSc/TJTfX63zpVI/AAAAAAAACOI/na3ZgyUFd2M/s1600/1227573326bxGKsyD.jpg