{"id":851,"date":"2017-02-01T14:00:21","date_gmt":"2017-02-01T21:00:21","guid":{"rendered":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/?p=851"},"modified":"2020-07-31T16:00:58","modified_gmt":"2020-07-31T22:00:58","slug":"cpk-hard-vs-specification-limits","status":"publish","type":"post","link":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/cpk-hard-vs-specification-limits\/","title":{"rendered":"Cpk and Hard vs Specification Limits"},"content":{"rendered":"<p>A customer called upset about Cpk. He had a runout of 0.010, but was getting a very low Cpk. Turns out he&#8217;d made the classic mistake of confusing a hard limit (e.g., zero) with a specification limit. I had him use QI Macros with an Upper Specification limit (USL) and no Lower Specification Limit (LSL). His Cpk immediately jumped to a more expected value of 1.78.<\/p>\n<p>Later in the day another customer asked why Cpk is calculated as the minimum of the upper or lower Cpk? Because you use the one closest to the average. I think that customer may have had the same problem, confusing a hard limit with a specification limit.<\/p>\n<p>If you have cycle time data, there is no lower specification. Zero is a hard limit, not a specification. If you specify an LSL, you will get a high Cp, but a Cpk less than one.<\/p>\n<div style=\"width: 660px\" class=\"wp-caption alignnone\"><img decoding=\"async\" class=\"size-full\" src=\"https:\/\/www.qimacros.com\/images\/blog\/one-sided-spec-incorrect.png\" alt=\"Using specification limits incorrectly\"  \/><p class=\"wp-caption-text\">Cpk when a hard limit confused with a specification limit<\/p><\/div>\n<p>In healthcare, for example, time to thrombolytics should be less than 90 minutes. There is no LSL. Use a USL of 90 to get a proper Cpk.<\/p>\n<div style=\"width: 660px\" class=\"wp-caption alignnone\"><img decoding=\"async\" class=\"size-full\" src=\"https:\/\/www.qimacros.com\/images\/blog\/one-sided-spec-limit.png\" alt=\"Histogram with one sided specification limit\" \/><p class=\"wp-caption-text\">Histogram with one sided specification limit<\/p><\/div>\n","protected":false},"excerpt":{"rendered":"<p>A customer called upset about Cpk. He had a runout of 0.010, but was getting a very low Cpk. Turns out he&rsquo;d made the classic mistake of confusing a hard limit (e.g., zero) with a specification limit. I had him use QI Macros with an Upper Specification limit (USL) and no Lower Specification Limit (LSL). [&hellip;]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,7],"tags":[195,26,222],"class_list":["post-851","post","type-post","status-publish","format-standard","hentry","category-qi-macros","category-six-sigma","tag-capability-analysis","tag-cpk","tag-one-sided-specification-limits"],"_links":{"self":[{"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/posts\/851","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/comments?post=851"}],"version-history":[{"count":6,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/posts\/851\/revisions"}],"predecessor-version":[{"id":3077,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/posts\/851\/revisions\/3077"}],"wp:attachment":[{"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/media?parent=851"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/categories?post=851"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.qimacros.com\/lean-six-sigma-blog\/wp-json\/wp\/v2\/tags?post=851"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}