Mar 5 2023
The literature on Quality does not dwell on its interactions with other components of a manufacturing system, like Production, Engineering, Production Control/Logistics, or Supply Chains. As a consequence, it is missing out on key relationships that affect the value of quality improvement.
Oct 4 2022
In his latest column in Quality Digest, Don Wheeler wrote the following blanket statements, free of any caveat:
Source: Wheeler, D. (2022) Converting Capabilities, What difference does the probability model make? Quality Digest
Michel Baudin‘s comments:
Wheeler’s first statement might have applied 100 years ago. Today, however, there are many models in probability that are not based on the assumption that data are “observations from a set of random variables that are independent and identically distributed”:
In his second statement, Wheeler seems determined to deter engineers and managers from studying probability. If a prominent statistician tells them it serves no useful purpose, why bother? It is particularly odd when you consider that Wheeler’s beloved XmR/Process Behavior charts use control limits based on the model of observations as the sum of a constant and a Gaussian white noise.
Probability models have many useful purposes. They can keep from pursuing special causes for mere fluctuations and help you find root causes of actual problems. They also help you plan your supply chain and dimension your production lines.
As Wheeler also says, “Many people have been taught that the first step in the statistical inquisition of their data is to fit some probability model to the histogram.” It’s time to learn something new, that takes advantage of IT developments since Karl Pearson invented the histogram in 1891.
Fitting models to a sample of 250 points based on a histogram is old-hat. A small dataset today is more 30,000 points, and you visualize its distribution with kernel density estimation(KDE), not histograms.
#donwheeler, #probability, #quality
Feb 23 2022
[Featured image: a Han dynasty bronze ruler]
As a general principle, in manufacturing, you need to do the work the same way every time if you want the output to be consistent. In some cases, like extracting metals from ores, you need to tweak processes to produce consistent output from raw materials of varying compositions. Then the tweaks themselves must be executed consistently so that the response to a particular variation in ore content is always the same.
Standards are an area where China had a 2,000-year headstart but neither the scientific nor the industrial revolutions occurred there.
Apr 16 2021
[The featured image is of a Vietnamese satellite undergoing final test in Japan]
As Jay Bitsack pointed out in his comments on LinkedIn about my previous post, the portability of a method from epidemiology to manufacturing quality is not a foregone conclusion. Formally, the logic of validating a vaccine seems applicable to the solution of a quality problem. They look similar when you consider only outcomes in terms of infection rates or the proportion of defectives.
There are differences between data sets from a clinical trial and tests run before and after a process change in production that may affect the applicability of a method. We examine the conditions for the approach developed by Carlo Graziani for vaccine efficacy to cross over to quality control. Then we work out the math of Graziani’s method and the means to apply it.
Apr 2 2021
Pfizer-BioNTech just announced the results of a COVID-19 vaccine clinical trial of 12- to 15-year olds. Because the vaccinated group had 0 infections, the news media jumped to the conclusion that the vaccine has “100% efficacy” on 12- to 15-year olds. That is what the chyron said on NBC news. A look at the published trial results and quick analysis with current methods confirms that the vaccine works for that population but not that it eliminates 100% of the infections.
Nov 13 2023
Process Capability Indices
Within the quality profession, a capable process is one with a high C_{p_k}. In the field, it is not quite so simple. My colleague Joerg Muenzing recently shared concerns about the process capability indices:
Let’s take a closer look.
Continue reading…
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By Michel Baudin • Data science • 5 • Tags: capability index, Cp, Cpk, Process capability, Quality