communicating your process capability - minitab · process capability allows us to assess and...
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Communicating Your Process Capability
Michelle Paret
Minitab Inc.
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“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
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Top EnhancementsWhat is Process Capability?
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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.
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Specification Limit(s)► Minimum and maximum acceptable values
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Capability Indices► Compare tolerance to process spread
Process spread
Tolerance
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Data Structure & Assumptions
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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
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Data Structure
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Assumptions
► We must check 2 critical assumptions:
1. Process is stable over time 2. Distribution is a good fit
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Assumptions
► Why does the distribution matter?
Indices are
calculated using
the distribution
(NOT the data)
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Normal curve with data from a
normal distribution
Assumptions
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Assumptions
Normal curve with data from a
Weibull distribution
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Examples:
Good or Bad?
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Cpk is actually much worse
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Process is unpredictable
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More than 1 population represented here
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Only Ppk is valid.And you can’t assess stability.
So, let’s talk about Cpk vs. Ppk…
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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”
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Within vs. Overall
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Last concept…
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Cp – can my car fit in the garage?
Cpk – am I headed towards a wall?
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Which Cp is better?
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Which Cp is better?
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Which Cpk is better?
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Which Cpk is better?
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When capability is poor, what strategy should you use to
improve it?
(based on Cpk and Cp)
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Improving process capability
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Want to learn more?
Instructor-led courses E-Learning with
Quality Trainer
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Top EnhancementsThank you!
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Reference Material