{"id":1164,"date":"2014-04-14T11:24:16","date_gmt":"2014-04-14T15:24:16","guid":{"rendered":"https:\/\/keyknowledge.com\/?p=1164"},"modified":"2014-04-14T11:24:16","modified_gmt":"2014-04-14T15:24:16","slug":"process-capability-and-process-control-how-are-they-different","status":"publish","type":"post","link":"https:\/\/keyknowledge.com\/de_ch\/2014\/04\/process-capability-and-process-control-how-are-they-different\/","title":{"rendered":"Prozessf\u00e4higkeit und Prozesssteuerung \u2013 worin unterscheiden sie sich?"},"content":{"rendered":"

In our Green Belt course I introduce the concepts of process capability and process control.\u00a0\u00a0 I find that many students have trouble understanding the difference between the two.\u00a0 This is compounded by the fact that the output of a process may fall into of four possible combinations of capability and control.
\nProcess capability looks at whether or not the output of the process is capable of satisfying the customer specification or requirement.\u00a0 \u00a0Process capability is usually measured using either the Capability Ratio (Cp) or the Capability Index (Cpk).\u00a0 A value of 1.0 for either Cp or Cpk implies that the spread of the process output is equal to the specification or requirement.\u00a0 Process spread in this case is defined as six standard deviations, with standard deviation being a statistic that measures variability.\u00a0 A minimum value of 1.33 for Cp and Cpk is desired, and the larger the value the better.\u00a0\u00a0 The formulas for Cpk consider how well the output of the process is centered within the specification; the formula for Cp does not.
\nIn order for us to calculate Cp or Cpk for a process, three conditions must be met.\u00a0 One, the output of the process must be a continuous variable.\u00a0 Two, the output of the process must be normally distributed.\u00a0 Three, the process output must be in a state of statistical control.
\nProcess control looks at whether or not the output of the process is in a state of statistical control.\u00a0 There is variation in the output of all processes, and this variation may be due to common causes or special causes.\u00a0 Common causes are inherent in the process, while special causes are assignable to a person or an event.\u00a0 The variation that is present in the output of the process, whether it is common cause or special cause, may or may not result in defects.
\nIf a process exhibits special cause variation, it is said to be out of statistical control.\u00a0 This implies that the output is not stable or predictable.\u00a0 \u00a0If a process exhibits common cause variation, it is said to be in a state of statistical control.\u00a0\u00a0 This implies that the output is stable and predictable.\u00a0\u00a0 The fact that a process is in control does NOT mean that the output of the process is normally distributed and no such inference should ever be made. \u00a0We determine whether or not a process is in control by using either a run chart or control charts, with control charts being the preferred method.
\nDr. W. Edwards Deming stated the following: \u201c\u2026the type of action required to reduce special causes of variation is totally different <\/b>from the action required to reduce variation and faults from the system itself\u2026\u201d in his book Out of the Crisis<\/i>, p. 309. \u00a0In order to make the right decision about how to improve a process, we must separate common cause variation from special cause variation.\u00a0 Special cause variation is reduced by eliminating the special causes.\u00a0 Common cause variation is inherent in the process and can only be reduced by changing the process.
\nTwo mistakes are therefore possible.\u00a0 One is to tell people to find and eliminate special causes when none exist.\u00a0 The other is to start making changes to the process when we should in fact be looking for special causes.
\nOnce we understand whether or not a process is capable and whether or not it is in control, we must next look at the two together.\u00a0 There are four possible combinations of capability and control, and the actions that are appropriate in each case are different.
\nThe most desirable state is a process that is in control and capable, where the output is stable and the output is meeting the requirements of the customer.\u00a0\u00a0\u00a0 Even if this is the case, further improvement is possible by centering the mean of the output of the process on the target value that provides the most satisfaction, and by reducing variation around the mean of the output so that the capability of the process is as high as possible.
\nAnother possible combination is a process that is in control but not capable.\u00a0 The first action should be to center the output of the process on the target value and then reevaluate to see if the output became capable.\u00a0 If the process is still not capable, then the process needs to be changed in order to reduce common cause variation and improve capability.
\nThe third possible combination is a process that is out of control but is capable.\u00a0 The appropriate action here is to find and eliminate the special causes of variation in order to bring the process under control.\u00a0\u00a0 Further improvement may still be possible and desirable.
\nThe fourth and final combination is the worst of the four, a process that is out of control and not capable. \u00a0\u00a0In this case the process should be centered on the target and the special causes of variation eliminated.\u00a0 At this point it should be reevaluated to see whether or not it is capable.\u00a0 Further improvement may still be possible and desirable.
\nIhre Kommentare und Fragen zu diesem Artikel sowie Anregungen f\u00fcr zuk\u00fcnftige Artikel sind herzlich willkommen. Sie k\u00f6nnen mich gerne per E-Mail kontaktieren unter roger@keyperformance.com<\/a>.
\nAbout the author:\u00a0 Mr. Roger C. Ellis is an industrial engineer by training and profession.\u00a0 He is a Six Sigma Master Black Belt with over 45 years of business experience in a wide range of fields.\u00a0 Mr. Ellis develops and instructs Six Sigma professional certification courses for Key Performance LLC.\u00a0 \u00a0For a more detailed biography, please refer to www.keyperformance.com<\/a>.
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In our Green Belt course I introduce the concepts of process capability and process control.\u00a0\u00a0 I find that many students have trouble understanding the difference between the two.\u00a0 This is compounded by the fact that the output of a process may fall into of four possible combinations of capability and control. Process capability looks at […]\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"cybocfi_hide_featured_image":"","footnotes":""},"categories":[1],"tags":[81,82,80],"class_list":["post-1164","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-process-capability","tag-process-control","tag-six-sigma"],"_links":{"self":[{"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/posts\/1164","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/comments?post=1164"}],"version-history":[{"count":0,"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/posts\/1164\/revisions"}],"wp:attachment":[{"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/media?parent=1164"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/categories?post=1164"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/keyknowledge.com\/de_ch\/wp-json\/wp\/v2\/tags?post=1164"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}