This document provides guidance on analyzing process stability and capability using statistical process control (SPC). It explains that processes must be stable before improvement efforts, and lists signs of instability like trends, patterns or excessive variation. It also defines process capability indices and targets manufacturers should aim for. Finally, it discusses sustaining improvements through monitoring control charts and reviewing process performance over time.

1. SPC
Simplified
How to Plug the Leaks
In Your Cash Flow!
SPC Made Easy
Quick Reference Card
For SPC Video
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Analyze Stability
Processes that are "out of control" need to be
stabilized before they can be improved using the
problem-solving process. Special causes require
immediate cause-effect analysis to eliminate
variation.
Thefollowingdiagramwillhelpyouevaluatestability
inanycontrolchart.Unstableconditionscanbeany
of the following:
Various trends are also considered unstable
conditions. For a complete list see qimacros.com/
qiwizard/stability.html. Any of these conditions
suggests that an unstable condition may exist.
Considerinvestigatingthespecialcauseofvariation.
The QI Macros automatically identify these for you.
Use the Ishikawa diagram to analyze potential root
causes.
Onceyou'veeliminatedthespecialcauses,youcan
turn your attention to using the problem solving
processtoreducethecommoncausesofvariation:
Analyze Capability
When Cp, the process capability index, and Cpk,
the centering index, are over 1.0, the process is
capable, but most manufacturers require
Cp>1.33.
Use the problem solving process to identify and
prevent the root causes of waste (usually below
the lower specification limit) and rework (usually
above the upper specification limit). The goal, is
to reduce variation so that all of your points fit
within the specification limits, clustered around a
target.
$3.00
Capable
Cp>1
Cpk>1
Cpk Sigma
1.0 3
1.33 4
1.66 5
2.0 6
BEFORE
AFTER
Problem
Solving
Lower
Spec
Limit
Upper
Spec
Limit
Waste Rework
Target
8
7
6
5
4
3
2
1
0
11
10
9
8
7
6
5
4
3
2
1
0
LSL USL
LSL USL
Time to Install (Before)
Time to Install (After)
3σ
-3σ
-1σ
-2σ
2σ
1σ
SPC
SIMPLIFIED
SPC Made Easy

2. Sustain the Improvement
Purpose: Stabilize and Sustain the Improvement
Key Tools
Flowchart: Show the flow of work
through a process including all
activities, decisions, and
measurementpoints.
Control Chart: Help analyze,
sustain, and monitor the current
levels of process stability and to
identify key issues for problem
solving or root cause analysis.
Histogram:Determinethecapability
(i.e., the level of performance the
customerscanconsistentlyexpect)
oftheprocessandthedistributionof
measurabledata.
SustainTheImprovement(Control)
FISH Step Activity
Focus 1 Refine the process
Define 2 Identify the critical to quality
Measure indicators(CTQs)
Improve 3 Implement the critical to quality
Analyze indicators
Improve
Sustain 4 Check the process for stability and
Control capability
Honor 5 Review, recognize, and refocus
Cycle
Time
Choosing The Right Chart
Variable
Data
To automate these charts, try the QI Macros
for Excel. Download a FREE 30 day trial at
www.qimacros.com/freestuff.html.
QI Macros Control Chart Wizard
The QI Macros can analyze your data and
select and run the right control chart for you.
Defect Data
To learn more about SPC, consider our one
hour training video: www.qimacros.com/
spc-simplified-training-video.html
QI Macros
Sample Data
XbarS
XbarR
XmR
Individuals
u Chart
c Chart
p Chart
Variable
(Measured)
decimal: 1.3
Attribute
(counted)
integer: 1,2
np Chart
Defects
People
Events
More than
one defect
per item
Defective
Defective?
Yes/No
How many defects?
Count Defective Items?
CountNumberof
Defects Per Item
Services &
Manufacturing
Manufacturing
Sample Size
Constant
defects/100
Varies
wrongorders/orders
medicationerrors
infections/patient days
Constant
injuries/month
Patient falls/month
Varies
errors/orders
components/assemblies
Patient falls/days
1
length or weight per item
time/project
cost/project
infections/1000 patient days
2-5
diameters,lengths
tensile strength
resistance
6-25
diameters,lengths
tensile strength
resistance
Samples used
to analyze large
runs of product
Time Cost
$Weight
Length
Height
X
X
X
X
Rates
or Ratios?
Individuals
Subgroups
Subgroups