STATISTICS

Statistics

Statistics

Statistical Quality Control:

Statistical quality control (SQC) techniques were developed to determine process capabilities and evaluate process performance. Process variability in SQC is distinguished as common cause versus special-cause variability. Common cause variability represents inherent process variation beyond the control of the worker. Special-cause (or assignable-cause) variability results from sources external to the process and is manifested as an abnormal deviation on a single trial or a sustained shift (instantaneous or developing trend) in the process. In SQC, the organization is responsible for identifying and eliminating special-cause variability.

Statistical process control (SPC) involves using statistical techniques to measure and analyze the variation in processes. Most often used for manufacturing processes, the intent of SPC is to monitor product quality and maintain processes to fixed targets.

SPC is used to monitor the consistency of processes used to manufacture a product as designed. It aims to get and keep processes under control. No matter how good or bad the design, SPC can ensure that the product is being manufactured as designed and intended. Thus, SPC will not improve a poorly designed product's reliability, but can be used to maintain the consistency of how the product is made and, therefore, of the manufactured product itself and its as-designed reliability.

Control Chart:

A primary tool used for SPC is the control chart, a graphical representation of certain descriptive statistics for specific quantitative measurements of the manufacturing process. These descriptive statistics are displayed in the control chart in comparison to their "in-control" sampling distributions. The comparison detects any unusual variation in the manufacturing process, which could indicate a problem with the process.

Several different descriptive statistics can be used in control charts and there are several different types of control charts that can test for different causes, such as how quickly major vs. minor shifts in process means are detected. Control charts are also used with product measurements to analyze process capability and for continuous process improvement efforts. Basic tools for SPC are:

1) Flow Charts

2) Run charts

3) Pareto Charts and analysis

4) Cause and effect diagram

5) Frequency Histogram

6) Control charts

7) Process capabilities studies

Causes and types of variation:

There is always some variation in every process; if you write your name ten times, your signatures will all be similar, but no two signatures will be exactly alike. There is an inherent variation, but it varies between predictable limits. If, as you are signing your name, someone bumps your elbow, you get an unusual variation due to what is called a "special cause". If you are cutting diamonds, and someone bumps your elbow, the special cause can be expensive. For many, many processes, it is important to notice special causes of variation as soon as they occur.

There's also "common cause" variation. Consider a baseball pitcher. If he has good control, most of his pitches are going to be where he wants them. There will be some variation, but not too much. If he is "wild", his pitches aren't going where he wants them; there's more ...