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Control Charts

Short-Run Statistical Process Control Techniques

Short production runs are a necessity in high-mix, low-volume manufacturing environments. The trend in manufacturing has been toward smaller production runs, with production runs – as well as products – tailored to the individual customer’s needs. Although this minimizes inventory and improves responsiveness to the customer, it complicates the application of statistical process control (SPC). Classical…

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Using Control Charts or Pre-control Charts

Every process falls into one of four states: Ideal: produces 100 percent conformance and is predictable Threshold: predictable but produces the occasional defect Brink of chaos: not predictable and does not produce defects Chaos: not predictable and produces defects at an unacceptable rate Processes tend to migrate toward chaos if not effectively managed. Pre-control Charts…

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Multivariate Control Charts: T2 and Generalized Variance

Multivariate analysis techniques may be useful in statistical process control (SPC) whenever there is more than one process variable. Multivariate control charting is usually helpful when the effect of multiple parameters is not independent or when some parameters are correlated. This article focuses on parameters that correlate when the Pearson correlation coefficient is greater than…

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A Guide to Control Charts

Control charts have two general uses in an improvement project. The most common application is as a tool to monitor process stability and control. A less common, although some might argue more powerful, use of control charts is as an analysis tool. The descriptions below provide an overview of the different types of control charts…

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Manage Project Performance with EVM and Control Charts

This article introduces the concept of earned value management (EVM) indexes, a project assessment technique, and control charts, a statistical tool for monitoring variation in a process, and describes how both may be used in tangent to capture more insight from project performance.  What to Know About EVM Indexes EVM is a project management technique…

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Help with a Future iSixSigma Article on Gage R&R

Hello iSixSigma readers! We are working on an article about gage R&R and need your help with data collection for analysis. What we need: Raw data itself (either 5-part or 10-part analyses including operators, parts, trials and measurements) The tolerance spread Is this one- or two-sided tolerance What we would like: Gage family information (e.g.,…

Specification Limits: Proceed with Caution

It’s natural for a manufacturing group to be concerned with a product’s specification limits – the upper and lower limits imposed on the process. Specification limits are sometimes designated by the producer, definitions of quality or, most frequently, a customer, in an attempt to narrow the distribution of a product’s properties. Unfortunately, to keep a…

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Should You Use a Mean or Individuals Control Chart?

To plot individual data or to group the data and plot the mean on a control chart, that is the question. Several authors (not including Shakespeare) have weighed in on this issue and I want to present their arguments and then my own. First, let us assume that the process conditions are such that using…

Recalculating Control Limits

From tedious time consuming task to opportunity for improvement. When applying control charts it is common practice to establish the control limits based on the process capability study and then use fix limits on the chart during production. This method is also frequently applied in SPC software programs. The advantage of using fixed limits is…

Why Control Chart Your Processes?

Control charting is a tool used to monitor processes and to assure that they remain in control or stable. Proposed by Walter Shewart in 1924, control charts help distinguish process variation due to assignable causes from those due to unassignable causes. Table 1: Types of Process Variation Found on a Control Chart Types of Variation Variation…

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From Quality Control to Quality Improvement

Everybody is familiar with control charts for quality control. An example of a control chart is shown below. In the example a packaging company who made blisters for the pharmaceutical industry found the process average for a critical characteristic was out of control. There were some ideas about possible causes but, as in most other…

When to Recalculate Control Limits

A problem that has often confronted practitioners using control charts is when to consider recomputing the control limits. I’ve asked this question to several experts and researched it in numerous SPC texts. Oddly, either the issue is ignored or there is no clear answer to this important question. So I’ll offer my recommendation in this…

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Make Valid Control Chart and Subgroup Assumptions

Six Sigma practitioners often state that Six Sigma is not about learning statistics, but is instead about understanding which tool to apply to each situation and how to properly interpret the results. We will attempt to understand the meaning of this statement in four real world examples I have experienced in industry. Control Charts Subgrouped…

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Non-normal Data Needs Alternate Control Chart Approach

Some practitioners mistakenly believe that it is not necessary to transform data before creating an individuals control chart when the underlying process distribution response is not normal. An individuals control chart, however, is not robust to non-normally distributed data. Therefore, it is important to use an alternate control charting approach. Necessary Transformation Consider a hypothetical…

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A Roadmap for Using Time-weighted Control Charts

Selecting the right type of control chart is a vital starting point for statistical process control (SPC). Which chart to use depends mainly on the classification of the data, the type of underlying distribution and the intent of the application. Selecting the wrong type can result in many false alarms, leading to expensive and fruitless…

Integrating SPC and SQC to Overcome Weaknesses in Each

Statistical quality control (SQC) and statistical process control (SPC) are two powerful tools, which have different goals and requirements for successful application. By using a methodology that combines the strengths of both approaches, it is possible to overcome the individual weaknesses of each one. The volume of calculations required by this technique prohibits manual data…

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The Impact of Control Strategies on Z Shift Values

When the principles of statistical process control (SPC) are used to manage a business or manufacturing process, any indication of the process going out of control will prompt some action on the part of the process owner. When it is well used, SPC will detect shifts in the process due to issues such as tool…

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