This is a crucial distinction that is frequently confused. Basically, specification limits have to do with the voice of the customer while control limits have to do with the voice of the process.
Nothing and everything. Though they are not directly linked, statistician and SPC expert Steven Wachs cautions that without evidence of process stability, capability data is useless.
Before applying statistical methods that assume normality, it is necessary to perform a normality test on the data.
It’s not subjective: Because it’s an objective measure of variation, standard deviation is an essential statistical tool for increasing quality.
Proper control chart selection is critical to realizing the benefits of Statistical Process Control.
Let’s take the confusion out of Cpk and Ppk.
The key is to specify a subgroup size so that significant shifts are detected with high probability and that insignificant shifts are unlikely to produce a signal.
The subgroup size formula and application are discussed in the conclusion of this two-part topic.
Many believe that an out of control process produces defective parts. That’s not always true.
While the construction of control charts is relatively straightforward, often a more difficult question is: how do I know what process characteristics to control in the first place?
Highly effective SPC programs combine technical competencies, such as using the right chart and sample size for the application, with good management principles such as ensuring operator involvement.
When Statistical Process Control is applied properly, tremendous benefits in profitability and process understanding are achieved.
In this case, we address $1.2 million worth of annual process variation.
By systematically detecting (and rectifying) sources of special cause variation upstream in the process, the important process outcomes become predictable.
More companies are leveraging high speed vision systems to inspect multiple quality characteristics on their products.
In order to maximize profitability while complying with government regulations regarding net package contents, food manufacturers and packagers must achieve an optimal balance.
While enormous dollars are spent reacting to problems, Quality teams can excel to new heights by focusing on prevention: of product failures, of scrap, and of other inefficiencies.
An approach to deploying control charts with short production runs is to utilize charts of common characteristics across different products.
Characteristics with larger nominal values tend to have more variation than characteristics with smaller nominal values.
Looking just at the auto industry, GM has recalled roughly 3 million vehicles this year in the U.S. alone. In 2009, GM recalled roughly 2.2 million vehicles.
Unfortunately, applying “simple” tools at the expense of tools with considerably more value (and really not much more complexity or difficulty) doesn’t cut it.
The purpose of control charting is to regularly monitor a process so that significant process changes may be detected.
Many manufacturing leaders believe that their production personnel use SPC properly, but the evidence suggests otherwise.
An OC curve allows practitioners to determine a sample size that will result in the detection of those process changes that are of practical significance while minimizing the occurrence of false alarm.
Yes, you can chart deviation-from-target. You do this by creating a Target chart, a chart that plots the deviation of subgroup values from a target value.
To answer that, we need to start off with what these capability indices represent.
As the bumper sticker says, “Shifts Happen” and so we generally want to use the slightly more liberal process capability of Cpk ≥ 1.33.
This article will review statistical hypothesis testing in general and then introduce equivalence testing and its application.
“Lean Manufacturing” (a.k.a. the Toyota Production System) is a toolbox of techniques to improve operational efficiency and eliminate waste.
Last month I began to answer this question by examining some of the 14 Principles of Lean Manufacturing according to Jeffery Liker. Let’s continue that examination.
Process Control is proactive rather than reactive. The key elements of process control are outlined in this article.
EWMA or Cusum charts are the best charts for detecting process shifts less than 1σ.