1 Colorado 5M WebEx Colorado 5M WebEx Variation, Run Charts, and Variation, Run Charts, and Control Charts Control Charts Beth A. Katzenberg, EdM, MBA, CPHQ Beth A. Katzenberg, EdM, MBA, CPHQ Director, Corporate Quality & Compliance Director, Corporate Quality & Compliance Colorado Foundation for Medical Care Colorado Foundation for Medical Care
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Colorado 5M WebExColorado 5M WebExVariation, Run Charts, and Variation, Run Charts, and Control ChartsControl Charts
Beth A. Katzenberg, EdM, MBA, CPHQBeth A. Katzenberg, EdM, MBA, CPHQDirector, Corporate Quality & ComplianceDirector, Corporate Quality & ComplianceColorado Foundation for Medical CareColorado Foundation for Medical Care
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Types of variationTypes of variation
Common causeCommon cause Always presentAlways present Inherent in processInherent in process Can predict Can predict
performance with a performance with a range of variationrange of variation
Cannot tell what Cannot tell what specifically causes specifically causes variationvariation
Special causeSpecial cause Abnormal, unexpectedAbnormal, unexpected Due to causes not Due to causes not
inherent in processinherent in process Can be identified Can be identified
(e.g., change in shift, (e.g., change in shift, weather, process)weather, process)
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You must understand the type You must understand the type of variation that is occurring of variation that is occurring
as this will determine how you as this will determine how you address the problem.address the problem.
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VariationVariation
Identify and study special cause If negative, minimize or prevent If positive, build into process
Special cause(unpredictable, unstable, out of control)
Change the process Do not react to individual differences or try to explain differences between high and low numbers
Common cause(predictable, stable, in control, inherent in process)
Appropriate action to takeType of variation
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PitfallsPitfalls
If only If only common cause variationcommon cause variation and treat and treat as special cause (tampering), leads to as special cause (tampering), leads to greater variation, mistakes, defectsgreater variation, mistakes, defects
If If common cause and special causecommon cause and special cause, and , and change the process, leads to wasted change the process, leads to wasted resources because the change won’t workresources because the change won’t work
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Tools to identify variationTools to identify variation
Run chart analysis: Run chart analysis: Common cause variation onlyCommon cause variation only
0
1
2
3
4
5
6
7
8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time
Common cause variation around the median:Common cause variation around the median: Only common cause variation present.Only common cause variation present. Output may or may not meet customer/ Output may or may not meet customer/ patient requirementspatient requirements
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Run chart analysis: RunsRun chart analysis: Runs
Run = one or more consecutive data Run = one or more consecutive data points on the same side of the medianpoints on the same side of the median
Excludes data points on the medianExcludes data points on the median
High probability High probability of special cause variation:of special cause variation:
Too few runsToo few runsToo many runsToo many runs
= 0.05)(
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Run chart analysis: Run lengthRun chart analysis: Run length
0
2
4
6
8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time
Special cause—run length:Special cause—run length:
<20 data points<20 data points (not on median): A run of (not on median): A run of 77 data points on one side of the median (either data points on one side of the median (either above or below) above or below)
20+ data points20+ data points (not on median): A run of (not on median): A run of 88 data points on one side of the mediandata points on one side of the median
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Run chart analysis: TrendsRun chart analysis: Trends
0
2
4
6
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time
Special cause—trends:Special cause—trends: Consecutive points all Consecutive points all going up or all going going up or all going down. May cross the down. May cross the median. Ignore 2+ median. Ignore 2+ consecutive points that consecutive points that are the same.are the same.
45 to 8
7101 or more621 to 10059 to 20
# Consecutive points all increasing or decreasingTotal # data points on chart
(Pyzdek, 2003)
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Run chart analysis: FreaksRun chart analysis: Freaks
0
2
4
6
8
10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time
Freaks:Freaks: The presence of more than one or The presence of more than one or two dramatic spikes suggests the process is two dramatic spikes suggests the process is out of control.out of control.
Run charts not as sensitive in identifying, Run charts not as sensitive in identifying, thus may fail to detect.thus may fail to detect.
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Run chart analysis: CyclingRun chart analysis: Cycling
Cycling:Cycling: A zigzag or saw-tooth pattern A zigzag or saw-tooth pattern with 14+ points in a row alternating up or with 14+ points in a row alternating up or down. down.
0
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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Run charts tipsRun charts tips
How many data points?How many data points? 15-20 minimum is preferable15-20 minimum is preferable
Median = 50%/50% split pointMedian = 50%/50% split point Precisely half of the data set will be above the Precisely half of the data set will be above the
median and half below itmedian and half below it
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Control chartsControl charts
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Control chartControl chart
Time
Qu
alit
y C
ha
rac
teri
sti
c
Low
High
UCL
An indication of a special cause
LCL
X
Run chart with control limitsRun chart with control limits
Determines type of variationDetermines type of variation
Is process stable? Predictable?Is process stable? Predictable?
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Dividing control chart into zonesDividing control chart into zones
Zone A
Zone B
Zone C
Zone C
Zone B
Zone A
Each zone is
1 sigma wide
UCL
LCL
X
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Identifying special causesIdentifying special causes
Apply independently to each side of the center Apply independently to each side of the center line:line: 1 point outside the 3 sigma limit1 point outside the 3 sigma limit 2 out of 3 consecutive points in zone A or beyond2 out of 3 consecutive points in zone A or beyond 4 out of 5 consecutive points in zone B or beyond4 out of 5 consecutive points in zone B or beyond <20 total data points:<20 total data points: 7 consecutive points in 7 consecutive points in
zone C or beyond on one side of center line zone C or beyond on one side of center line 20+ total data points:20+ total data points: 8 consecutive points in 8 consecutive points in
zone C or beyond on one side of center linezone C or beyond on one side of center line(continued)(continued)
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Identifying special causes, cont.Identifying special causes, cont.
Apply this test to entire chart:Apply this test to entire chart: <21 total data points:<21 total data points: 6 or more points in a 6 or more points in a
row steadily increasing or decreasingrow steadily increasing or decreasing 21+ total data points:21+ total data points: 7 or more points in a 7 or more points in a
row steadily increasing or decreasingrow steadily increasing or decreasing 14 consecutive points alternating up and 14 consecutive points alternating up and
down in saw-tooth patterndown in saw-tooth pattern 15 consecutive points in zone C (above and 15 consecutive points in zone C (above and
below center line)below center line)
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Deciding which control chart to useDeciding which control chart to use
Decide on type of data
Continuous(Variables , measurement ) data(Values on continuous scale ; e.g., time, temperatures , cost)
Attributes (count , discrete) data
(Values in discrete categories ; e.g., % waste, # falls, # errors,
% incomplete charts)
More than one observation per
sub-group ?NoYes
Fewer than 10 observations
per sub -group ?
Are there equal areas of opportunity ?
Are the subgroup
sizes equal?
Can both occurrences and non-occurrences
be counted?
NoYes
X-R chart X -S chart XmR chart
NoYes
No NoYes Yes
c-chartp-chartnp-chart u-chart
Average & range chart
Average & standard deviation
(sigma) chart
Individual & moving range
chart
Source: Carey, R. C. and Lloyd, R. C. Measuring Quality Improvement in Healthcare: A Guide to Statistical Process Control Applications, 1995
(From: Carey, R. G. & Lloyd, R. C. Measuring Quality Improvement in Healthcare
Out of control, unpredictable
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Just because a process is under control Just because a process is under control (common cause variation only), it does (common cause variation only), it does not mean that the process is meeting not mean that the process is meeting
expectations. expectations.
It just means that the process is It just means that the process is predictable and you are getting predictable and you are getting
consistent performance.consistent performance.
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Control charts tipsControl charts tips
Control limits are not specifications limits Control limits are not specifications limits (specification limits related to customer (specification limits related to customer requirements)requirements)
After removing special causes and recalculating After removing special causes and recalculating chart, continue to plot new data on this chart, chart, continue to plot new data on this chart, without recalculating control limits.without recalculating control limits. Recalculate control limits only when a permanent, Recalculate control limits only when a permanent,
desired change has occurred in the process and desired change has occurred in the process and only using data only using data afterafter the change occurred the change occurred
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Share the dataShare the data
Team meetingsTeam meetingsPost in break-roomsPost in break-roomsNewslettersNewslettersIntranetIntranet
ReferencesReferencesCarey, R.G. & Lloyd, R.C. Carey, R.G. & Lloyd, R.C. Measuring Quality Improvement in Healthcare: A Guide to Measuring Quality Improvement in Healthcare: A Guide to
Statistical Process Control Applications, Statistical Process Control Applications, Quality Resources, 1995.Quality Resources, 1995.
Pyzdek, R. Pyzdek, R. The Six Sigma Handbook: A Complete Guide for Green Belts, Black Belts, and The Six Sigma Handbook: A Complete Guide for Green Belts, Black Belts, and Managers at All Levels, Managers at All Levels, 2003.2003.
The Six Sigma Memory Jogger II, The Six Sigma Memory Jogger II, GOAL/QPC, 2002.GOAL/QPC, 2002.
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Beth Katzenberg, EdM, MBA, CPHQBeth Katzenberg, EdM, MBA, CPHQDirector, Corporate quality & complianceDirector, Corporate quality & complianceColorado Foundation for Medical CareColorado Foundation for Medical Care