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Communicating Your Process Capability
Michelle Paret
Minitab Inc.
Top Enhancements
“Statistics summarize everything
down to 1 number.
If data have more than 1 story to
tell, how can 1 summary statistic
do the job?”
Scott Hindle, Process Capability, QualityDigest.com, 29 August 2016
Top EnhancementsWhat is Process Capability?
Process Capability
Allows us to assess and quantify how well a product or service meets specifications
Many applications:• Manufactured parts
• Food products
• Call center hold times
• ER wait times
• Etc.
Specification Limit(s)► Minimum and maximum acceptable values
Capability Indices► Compare tolerance to process spread
Process spread
Tolerance
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Data Structure & Assumptions
How was it collected?
► In what order?
Enter data in chronological order.
►Were the data collected in subgroups?
Subgroup column in worksheet is optional.
Data Structure
Data Structure
Assumptions
► We must check 2 critical assumptions:
1. Process is stable over time 2. Distribution is a good fit
Assumptions
► Why does the distribution matter?
Indices are
calculated using
the distribution
(NOT the data)
Normal curve with data from a
normal distribution
Assumptions
Assumptions
Normal curve with data from a
Weibull distribution
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Examples:
Good or Bad?
Cpk is actually much worse
Process is unpredictable
More than 1 population represented here
Only Ppk is valid.And you can’t assess stability.
So, let’s talk about Cpk vs. Ppk…
Within vs. Overall► Cpk
• Only accounts for WITHIN subgroup variability
• Does not account for the shift and drift between subgroups
• Is called potential capability because it represents the potential to produce parts within spec, presuming no variation between subgroups (i.e. over time)
► Ppk• Accounts for OVERALL variability across all measurements
• Includes the variation within subgroups AND the shift and drift between them
• Where you are “at the end of the day”
Within vs. Overall
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Last concept…
Cp – can my car fit in the garage?
Cpk – am I headed towards a wall?
Which Cp is better?
Which Cp is better?
Which Cpk is better?
Which Cpk is better?
When capability is poor, what strategy should you use to
improve it?
(based on Cpk and Cp)
Improving process capability
Want to learn more?
Instructor-led courses E-Learning with
Quality Trainer
Top EnhancementsThank you!
Reference Material
