Statistical Quality Control

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Statistical Quality Control (SQC) is the term used to describe the set of statistical tools used by quality improvement professionals to monitor, control, and improve products and processes.

Statistical Quality Control Basics

SQC tools fall into three broad categories...

1. Descriptive Statistics which are used to describe quality characteristics and relationships. Included are statistics such as the mean, standard deviation, the range, and other measures of the distribution of data.

2. Statistical Process Control (SPC) which involves inspecting samples of the output from a process and deciding whether the process is producing products with characteristics that fall within a predetermined control limits. SPC answers the question of whether the process is functioning in a stable and predictable manner or not. Low cost SPC training.

3. Acceptance Sampling which is the process of randomly inspecting a sample of goods and deciding whether to accept the entire batch based on the results. Acceptance sampling determines whether a batch of goods should be accepted or rejected.

descriptive statistics

The tools in each of these categories provide different types of information for use in analyzing quality. Descriptive statistics are used to describe certain quality characteristics, such as the central tendency and variability of observed data. Although descriptions of certain characteristics are helpful, they are not enough to help us evaluate whether there is a problem with quality. 

Acceptance sampling can help us do this. Acceptance sampling helps us decide whether our desired quality level has been achieved for a batch of products, and therefore whether to accept or reject the items produced.


Although this information is helpful in making the quality acceptance decision after the product has been produced, it does not help us identify and catch a quality problem during the production process. For this we need tools in the Statistical Process Control (SPC) category.

All three of these statistical quality control categories are needed to help us in measuring and evaluating the quality of products or services. However, statistical process control (SPC) tools are the most value-add because they identify quality problems during the production process.




Variation in processes leads to quality defects and lack of product consistency. No two products are exactly alike because of slight differences in materials, workers, machines, tools, and other factors. This variation is called common, or random, causes of variation.

Common Cause Variation

Common causes of variation are based on random causes that we cannot, or choose not, to control. This type of variation is unavoidable and is due to slight differences in processing. An important task in Six Sigma and quality improvement is to find out the range of natural random (common cause) variation in a process.

Assignable Cause Variation

The second type of variation that can be observed in processes involves variation where the causes can be precisely identified and eliminated. These are called assignable causes of variation. An example of this type of variation is a poor batch of raw material or a piece of equipment in need of repair.

In each of these cases the problem can be identified and corrected. If these problems are allowed to persist, they will continue to create a problems in the quality of the product.

Statistical Process Control

Statistical Process Control Basics

Statistical Process Control

Control Charts

Control Chart Selection

Control Limits

Capability Indices - Cpk, Etc.

Process Capability Study

Statistical Process Control Control Charts

Xbar & R (Range) Chart

Xbar & s (Standard Deviation) Chart

I (Individuals) & MR (Moving Range) Chart

p Chart

np Chart

u Chart

Learn more about Statistical Quality Control here.

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