Control charts a short journey
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This document discusses distinguishing between common cause and assignable cause variation in processes. Common cause variation is always present and can have many different causes, representing the natural variation of a process. Assignable cause variation occurs when an unusual event happens. Understanding the type of variation allows you to make the right decisions and respond appropriately, often through the use of control charts. Control charts have two parts - an individuals chart that shows the center and a moving range chart that shows the spread. Shewhart created rules to detect variation using control charts.
Control charts a short journey
- 1. EVERY PROCESS HAS VARIATION Shewhart designed a method to distinguish between two types of variation: COMMON CAUSE ASSIGNABLE CAUSE This is when an unusual event happens, good or bad. This is always present and can have lots of different causes. It is also called the natural variation of the process and can be: LARGE or small. srnP Attributes data OR Variable data • rn1NK and investigateOBSERVE • g "th "Today I counted 3 red cars" "My journey takes 1-2 hours" PRocErn usm e1 er: 4111'4111'4111' CE)CE) 0 By understanding what type of variation your process has, RIGHT you can make the RIGHT decision and respond appropriately. DECISIONS MOST COMMON CONTROL CHART The most commonly used Control Chart is the Individuals and Moving Range chart. The chart has 2 PARTS. INDIVIDUALS The individuals chart provides information about the CENTRE MOVING RANGE The moving range chart is about the S P R E A D _:.,. . ' . . . . . . . . . . .. . .. .. . . . . .. . . � . .. . "On TARGET with minimum variation" KEEP CALM AND Shewhart created four detection rules called the "WESTERN ELECTRIC FOLLOW THE RULES RULES", of which rules 1 and 4 are the most commonly used. m Western Electric Rule Ill Western Electric Rule . . ------IA------------ One pomt 1s ____ �-t---------- 8 points or more in a row, either above or below the centre. outside either the ::_-._::_-_-_ ;,�r---_::;UPPER or LOWER -. =========--=;7�:-=:: control limit -------------�---- CONTROL LIMITS The control limits are positioned at 3 standard deviations from the centre. Lower Centre of Upper control limits the process control limits -- -- 90-88% 99-100% If your process ONLY contains common cause variation, you can make predictions about the future performance of that process. THE FUTURE IS BRIGHT! 3 4 5 8 WEB RESOURCES Click to Access � Guide: The Process Manager Plus � Data Analysis Package � How to Use Control Charts webinar CONTACT US Call: +44(0)1676 522766 www.pmi.co.uk Copyright 1992 -2016 Process Management International Ltd. All rights reserved.