SPC Control Limits

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SPC Overview


Statistical Process Control (SPC) Charts with Control Limits were first developed by Dr. Walter Shewhart in the early 20th century in the U.S.  Control Charts are a graphical and visual plot of a process and is charted over time like a Time Series Chart.  

From a visual management aspect, a Time Plot is more powerful than knowledge of the last measurement. Control Charts are meant to indicate change in a process. All SPC charts have a Central Line and Control Limits to aid in the identification of Special Cause variation.


Statistical Process Control (SPC) involves the use of statistical techniques, to interpret data, to control the variation in processes. SPC is used primarily to act on out of control processes, but it is also used to monitor the consistency of processes producing products and services.

A primary SPC tool is the Control Chart - a graphical representation for specific quantitative measurements of a process input or output.  In the Control Chart, these quantitative measurements are compared to decision rules calculated based on probabilities from the actual measurement of process performance.

Control Charts provide you with two basic functions; one is to provide time based information on the performance of the process which makes it possible to track events affecting the process and the second is to alert you when Special Cause variation occurs.

And two; Control Charts graphically highlight data points that do not fit the normal level of variation expected. It is standard that the Common Cause variation level is defined as +/- 3 Standard Deviations from the Mean. This is also know as the UCL and LCL respectively. It’s all based off probabilities.

Control Limits


Purpose

Control Limits are calculated values and lines plotted on a Control Chart. They are used to determine the state of statistical control of a process. The Upper and Lower Control Limits are generally equal to the Mean (+ or -) plus or minus three Standard Deviations, respectively. If a process point exceeds either the UCL or the LCL, the process is considered to be out of control, and action should be taken.

Anatomy

SPC Control Limits Example

Reference: Statistical Process Control – Ford/GM/Chrysler

Terminology

A. Upper Control Limit (UCL) – The upper range of process control. The UCL is by convention calculated to equal to the process Mean plus three Standard Deviations.

B. Center Line – Calculated as the process Mean over the period being investigated

C. Lower Control Limit (LCL) – The lower range of process control. The LCL is by convention calculated to equal to the process Mean minus three Standard Deviations.

D. Plot of process sample statistic in chronological order vs. sample number. Any excursion in this plot above the UCL or below the LCL represents an out-of-control condition and should be investigated.
E. Out of Control Point – A single process point showing the most obvious sign of an Out-of-Control situation, i.e. being beyond either the UCL or LCL.

Specification limits should not be shown on a Control Chart because of the confusion often generated. Control Charts and their limits are the Voice of the Process while specification limits are the Voice of the Customer. The UCL and LCL indicate when a process is “out of control” or exhibiting Special Cause variation but NOT WHY!

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

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