communicating your process capability - minitab · process capability allows us to assess and...

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