# Category: Control Charts

## Common Cause vs. Special Cause Variation: What’s the Difference?

Published:What is Common Cause Variation? Common cause variation is the kind of variation that is part of a stable process. These are variations that are natural to a system and are quantifiable and expected. Common cause variations are those that are predictable, ongoing, and consistent. Major changes would typically have to be made in order […]

Read more »## Think Outside the Box Plot

Published:A box plot may seem like a simple tool, but as this example shows it can help reveal common and special cause variation that may not have otherwise been noticed. Be sure you’re looking below the surface to optimize your performance!

Read more »## U Chart

Updated:A U chart is one of four quality control charts that engineers use to track variation in a manufacturing process. The other three typical quality control charts are the p chart (which plots the proportion of defective items, the c chart (which shows the number of defects), and the np chart (which shows the number […]

Read more »## Short-Run Statistical Process Control Techniques

Published: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 […]

Read more »## Run Charts: A Simple and Powerful Tool for Process Improvement

Published:The run chart is a powerful, simple and easy-to-use process improvement tool. Often, the run chart is shortchanged as the statistical tests that can be used with run charts are overlooked. This article takes the reader through the benefits of a run chart as well as how to correctly create and analyze one. A process […]

Read more »## Using Control Charts or Pre-control Charts

Published: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 […]

Read more »## Manage Control Limits When Implementing Statistical Process Control

Published:Statistical process control (SPC) is the application of statistical methods to identify and control the special cause of variation in a process. Control charts, in theory, are used in product and process development to analyze processes. When a process is shown to be in control in both an average and range chart the process can […]

Read more »## Multivariate Control Charts: T2 and Generalized Variance

Published: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 […]

Read more »## Go Beyond Control Chart Limitations to Predict and Improve Processes

Published:Article authors, participants in discussion forums and even software documentation claim that any data and especially time-sequenced data are useful only if they are homogeneous – that is, only if they display control chart stability (CCS).1,2,3,4 According to one quality control expert, Donald J. Wheeler, when out-of-control signals occur, “all of our computations, and all of […]

Read more »## Leverage Six Sigma to Manage Operational Risk in Financial Services

Published:The recent spate of events such as rogue trading losses, flash trades and a seeming outbreak of Ponzi schemes has made many in financial services wary about their existing risk management practices. In post-financial crisis times, financial services companies are already under pressure to cut their operational costs while also being expected to comply with […]

Read more »## The Complete Guide to Understanding Control Charts

Published: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 […]

Read more »## Manage Project Performance with EVM and Control Charts

Published: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 […]

Read more »## Understanding Statistical Process Control [VIDEO] – With Eduardo Santiago

Published:Statistical process control (SPC), despite sounding esoteric, is a subject that every process owner and worker should – and can – understand, at least at a high level. Knowing whether a process is in control and stable is paramount to producing a product or service that meets customer needs. In this hour-long Minitab training course […]

Read more »## Specification Limits: Proceed with Caution

Published:A true control strategy should be aligned with the process’s statistically based control limits.

Read more »## Should You Use a Mean or Individuals Control Chart?

Published: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 […]

Read more »## Recalculating Control Limits

Published: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 […]

Read more »## Why Control Chart Your Processes?

Published: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 […]

Read more »## From Quality Control to Quality Improvement

Published: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 […]

Read more »## When to Recalculate Control Limits

Published: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 […]

Read more »## Make Valid Control Chart and Subgroup Assumptions

Published: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 […]

Read more »## Non-normal Data Needs Alternate Control Chart Approach

Published: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 […]

Read more »## A Roadmap for Using Time-weighted Control Charts

Published: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 […]

Read more »## The Impact of Control Strategies on Z Shift Values

Published: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 […]

Read more »## Integrating SPC and SQC to Overcome Weaknesses in Each

Published: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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