Process capability
Premiered by EDOUARD GATETE of
The Pennsylvania State University.
 Definition
 Measurements of process capability
 How can Process Capability be used in an
 A real world example
 Exercise
 Summary
 Process capability is defined as a how
likely a product is going to meet the design
– It has been argued whether the specifications are
actually set by customers or design engineers.
Measurements of process
 Most of the times,
conventional process
capability work quite well, but we have to
keep in minds that they will be times when
the usual formulas don’t work.
– An index that measure potential process capability
is used in unusual situation.
Measurements of Process
 The formula C sub P is the tolerance
divided by six standard deviations (also
known as sigma)
» C sub P = tolerance/6-sigma
Reveals how many six-sigma fits within a specified
In order for a product to meet specifications, at least
99.773% of productions should confirm anticipated
operation condition.
Measurement of process
 Population : a collection of all observation
of interest to a decision maker.
 Sample: a portion of population
 population distribution: a distributions of
the entire population
 sampling distribution: distributions of
sample means.
How to use process capability in
an organization
 The control chart
• bell-shaped curve: as long as all inputs to a
process remain constant, the output will
always vary
The difference between Stable
and capability
 A process is capable if a given product
meets specifications.
 A process is stable if there is a common
variation present in the process
The disadvantage of using
process capability
 Over-correcting:
the most common
 reducing variation
Factors influencing the
tolerance allocation
 Statistical aspects of the dimensions and
manufacturing processes involved
 Assembly requirements and conditions
 the cost-tolerance relationship
 International Standards Organization's
practices for fits.
A real world example
 Taking an example of a car manufacture
company. The process capability will be
used if out of 1000000 cars produced, only
3.4 cars come out defective
 A certain manufacturing process is
producing metal washers that have
diameters that are known to be normally
distributed with mean 1 inch and standard
deviation .006 inch. The process is known
to be stable and in control. The
specifications for these washers are 1 inch
+/- .01. What percentage are within
specification limits?
 We have just reviewed process capability.
This process is used in production to
determine whether a product meets
specification on a consistent basis. Even
though they are a lot of complex ways to to
figured out process capability, it’s more
important to understand the basics before
we can get in more details.
 This presentation has definitely focused on
the basics of process capability. It is very
important to minimize defects in
production if you want to optimize the
 A bibliography used to come up with these
slides is as follow;
 Hart, M.
Quality Tools for Decreasing
Variation and Defining Process Capability.
V33 n2. Second Quarter 1992, p.6-11.
 Raouf, A. On developing optimal process
capability index. International Journal of
Quality & Reliability Management. V9 n7.
1992. P. 17-22
 Bothe, D. R. Measure process capability
when Cp won’t work. Quality. V34 n8. Aug
1995. P. 20-22
 Foster, T. S. An integrative approach.
Managing Quality 2001. P. 374-376.