Process Capability
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A PROCESS is a unique combination of tools, materials, methods, and
people engaged in producing a measurable output; for example a
manufacturing line for machine parts. All processes have inherent
statistical variability which can be evaluated by statistical methods...
The Process Capability is a measurable property of a process to the
specification, expressed as a process capability index (e.g., Cpk or Cpm) or
as a process performance index (e.g., Ppk or Ppm). The output of this
measurement is usually illustrated by a histogram and calculations that
predict how many parts will be produced out of specification.
Process capability is also defined as the capability of a process to meet
its purpose as managed by an organization's management and process
definition structures ISO 15504.
Process Capability looks at short term capability and long term
performance of a process with regard to customer specifications.
Two parts of process capability are: 1) Measure the variability of a
process, and 2) Compare that variability with a proposed specification or
product tolerance.
Process Capability
•
process control - refers only to the “voice of the process” - looking at the
process using an agreed performance measure to see whether the
process forms a stable distribution over time.
• process capability - measures the “goodness of a process” - comparing
the voice of the process with the “voice of the customer”. The voice of the
customer here is the specification range (tolerance) and/or the nearest
customer specification limit.
When to use it
• Use it when setting up a process, to ensure it can meet its specification
limits.
• Use it when setting specification limits, to ensure they are neither too wide
nor too narrow.
• Use it when investigating a process that is not meeting its specification
limits.
• Use it only when the process is stable and has a Normal distribution.
The results of most processes will vary around a central value and the 'capability'
of the process is defined as the spread of results around this value, with high
capability occurring when process results group closely around it.
The most common measure of this spread is standard deviation, and 'Process
Capability' may be defined as the range between three standard deviations either
side of the average.
Process Capability
• Process capability is the ability of a process to
consistently meet specified customer-driven
requirements
• Specification limits are set by management in
response to customers’ expectations
• The upper specification limit (USL) is the
largest value that can be obtained and still
conform to customers’ expectations
• The lower specification limit (LSL) is the
smallest value that is still conforming
Estimating Process Capability
• Must first have an in-control process
• Estimate the percentage of product or service
within specification
• Assume the population of X values is
approximately normally distributed with mean
and standard deviation
Process Capability
How
a
much
variability
b
How
centred
What it is
compares
spec range
(tolerance)
to process
width
how close
process
centre is to
nearest
spec limit
1
2
CAPABILITY
PERFORMANCE
Short Term
In Control
Pooled std dev
Long Term
Not in Control
Total / overall std dev
Cp
Pp
Cpk
Ppk
Cp = Process Capability. A simple and straightforward indicator of process capability.
Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered
distribution.
Pp = Process Performance. A simple and straightforward indicator of process
performance.
Ppk = Process Performance Index. Adjustment of Pp for the effect of non-centered
distribution.
Cp = USL-LSL
Pp = USL-LSL
6sP
6sT
sP = pooled standard deviation
sT = total standard deviation
Cpk is an index (a simple number) which measures how close a process is running to
its specification limits, relative to the natural variability of the process. The larger the
index, the less likely it is that any item will be outside the specs. A Cpk of 1.33 [4
sigma] or higher satisfy’s most customers
Pp, Ppk Process Performance Index basically tries to verify if the sample that you
have generated from the process is capable to meet Customer requirements. It differs
from Process Capability in that Process Performance only applies to a specific batch
of material.
Process Performance generally uses sample sigma in its calculation; Process
capability uses the process sigma value determined from either the Moving Range,
Range, or Sigma control charts.
Cp = (USL - LSL)/6*Std.Dev
Cpl = (Mean - LSL)/3*Std.dev
Cpu = (USL-Mean)/3*Std.dev
Cpk = Min(Cpl,Cpu)
Use as a rule of thumb the following chart
Red (Bad)
Yellow (OK)
Green (Good)
Cp
Cpk
Pp
Ppk
Sigma
< 1.00
< 1.00
< 1.33
< 1.33
< 4.5
1.00 - 1.33
1.00 - 1.33
1.33 - 1.67
1.33 - 1.67
4.5 - 5.5
> 1.33
> 1.33
> 1.67
> 1.67
> 5.5
(Cpk x 3) +1.5 = sigma
There are three types of measurement required:
• Internal measures
• Output (or quality) measures
• Satisfaction measures
For each measurement you select, you need to define:
• what it is (a precise definition)
• how you will gather the data (including sample sizes)
• how often you will gather the data
• how often you will report and review the data (including the
format in
• which the data will be presented)
• any targeted levels of performance (if known)
• who is responsible for measurement
Data collection tools include:
• Checksheets (Tallysheets)
• Concentration Diagrams (pictorial Checksheets)
• Traveller Time-logs
• Surveys/Questionnaires
• Interviews/Focus Groups
Process Capability
Example
You are the manager of a 500-user Operator. You
have instituted a policy that all calls must be
completed within ten minutes or less. For seven
days, you collect data on five calls per day.
Compute an appropriate capability index for the
delivery process.
Process Capability:
Example Solution
n5
X  5.813
R  3.894
d2  2.326
USL  X
10  5.813
CPU 

 .833672
3(R / d2 ) 3(3.894 / 2.326)
Since there is only the upper specification limit, we need to
only compute CPU. The capability index for the calls process
is .8337, which is less than 1. The upper specification limit
is less than 3 standard deviation above the mean.