Intro: Welcome to the E6S-Methods podcast with Jacob and Aaron, your source for expert training, coaching, consulting, and leadership in Lean, Six Sigma, and continuous improvement methods. In this episode number 67, we introduce the jargon used to describe process capability, with capability indices, in part 3A of this “In the Eye of the Cash-holder” series. What’s your Cpk? Here we go. http://bit.ly/E6S-067
Objection 1: In spec is good enough. Variation doesn't matter.
Counter 1: Any variation from target results in depleted function of your customer. Although customer specs are wide, customers often have to account for supplier variation in some other way, and end up releasing lower quality goods into the market. Variation is evil according to taguchi, not just to customers, but to society as a whole.
Objection 2: Cpk and other fancy jargon is on its way out.
Counter 2: Perhaps, but new jargon will surely take its place to describe process variation relative to outcome expectations
Objection 3: This is old stuff. Everyone knows this. Why are we bothering still?
Counter 3: It is true that these topics are almost 100 years old. Yet still, many industries still have processes without specifications (business processes, etc), and the idea of measuring process variation is still new and novel.
****Capability is in the eye of the Cash-holder***
Process Capability Indices
a. Fairly easy to calculate. Just a ratio between 6* Stdev and the specification range
i. Can be completely out of spec but still have good Cp (uncentered process). Ignores location.
b. Equation ;
c. Gives indication of how well a process could be (entitlement) if it were centered.
d. Cp=1 means the specification window is equivalent to a +/- 3 sigma process variation spread. i.e. 6 process sigma could fit into the specification range/window.
e. Cp is considered the “short-term” measure while Pp is “long-term”
i. Cp calculated by approximating short-term variation using the average ranges between each data point.
ii. Pp calculated using the entire spread of the data set.
iii. Most existing data is between short-term and long-term
1. To evaluate true short term data, consult local engineer/SME and ensure sub-grouping strategy is rational
2. Low volume (frequency) production, long cycle times or expensive/difficult-to-sample processes may not be able to be studied short-term
a. More hyperbolic. Choose the worst of 2 sides of the mean.
b. Equation ;
c. Cpk=1 means the process mean is centered in the specification is 3 standards deviations away from either specification limit. i.e. (3 sigma process)
i. This used to be the baseline definition of “capable.”
ii. Most customers expect Cpk>1.33 now (4 Sigma process)
d. Better indicator of actual pain than Cp or Pp alone. (location and spread considered). Cp is a good estimate of “entitlement,” establishing a reasonable improvement goal while Cpk & Ppk is an estimate of actual capability.
III Benefits of these indices: Can be approximated without statistical software. Was designed to be calculated by hand by process operators.
IV Pitfalls of these indices: More recently falling out of favor. Over-simplifies process behavior into a single number.
V Embarrassing story about Cpk done wrong, when Aaron was a (naive) co-op.
a. Excel template, weekly gathering of process data
b. Cherry picking, plotting the averages
c. Substituting numbers out to make the Cpk come out good. (tripling averages if they are the average of 3 parts.)
Outro: Thanks for listening to episode 67 of the E6S-Methods Podcast. Stay tuned for episode number 68 where we finish off our discussion of the capability indices, in part 3B of this “In the Eye of the Cash-holder” series. If you would like to be a guest on podcast, contact us through our website or tweet us @e6sindustries. Join our discussions on LinkedIn. Subscribe to past and future episodes on iTunes or stream us live on-demand with Stitcher Radio. Leave a 5-star review while you’re there. Find outlines and graphics for all shows and more at www.E6S-Methods.com. “Journey Through Success”