{"id":385,"date":"2013-09-27T21:02:49","date_gmt":"2013-09-27T20:02:49","guid":{"rendered":"https:\/\/blogs.bmj.com\/adc-archimedes\/?p=385"},"modified":"2015-05-19T21:31:47","modified_gmt":"2015-05-19T20:31:47","slug":"statsminiblog-bland-altman-plots","status":"publish","type":"post","link":"https:\/\/blogs.bmj.com\/adc\/2013\/09\/27\/statsminiblog-bland-altman-plots\/","title":{"rendered":"StatsMiniBlog: Bland Altman Plots"},"content":{"rendered":"<p><a href=\"https:\/\/blogs.bmj.com\/adc\/files\/2013\/08\/Image78.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"alignleft size-full wp-image-674\" src=\"https:\/\/blogs.bmj.com\/adc\/files\/2013\/08\/Image78.gif\" alt=\"StatsMiniBlog\" width=\"180\" height=\"76\" \/><\/a>Measuring things is what we do lots of, and we often want to measure things with a new machine. New, faster, shinier, cheaper, less invasive or more colourful &#8230; but we are almost always sold it as being highly correlated with the reference standard (p&lt;0.001).<\/p>\n<p>Think &#8211; what is this correlation and p-value telling us?<\/p>\n<p>Well, the <a title=\"StatsMiniBlog: Significance tests. Step three.\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/07\/17\/statsminiblog-significance-tests-step-three\/\">correlation tells us <\/a>how much one thing changes when the other does. The p-value test the hypothesis &#8220;what is the liklihood if these two things are just related by chance?&#8221;. Now, if we&#8217;ve got a lab blood glucose and a glucometer, would there ever be any chance at all of a p-value showing the two values might just be related by chance?<\/p>\n<p>What these values don&#8217;t tell us is what we want to know &#8212; when I get a reading on my shiny handheld non-invasive blood glucose tricorder, how far might that value be away from the real value I&#8217;d get from a proper laboratory test?<!--more--><\/p>\n<p>For this, we really want to see what are the &#8216;limits of agreement&#8217;: what is the 95% likely range of difference between one reading and the next. That&#8217;s what you&#8217;ll be wanting to look at a Bland-Altman plot for&#8230;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter\" src=\"http:\/\/i.stack.imgur.com\/0ZicM.png\" alt=\"\" width=\"682\" height=\"682\" \/><\/p>\n<p>This plot tells you for a reading of Device 1, what would the differences between the two measurement Devices be, with both an average difference &#8211; central line\u00a0&#8211; and the 95% confidence interval of the difference &#8211; outer\u00a0lines. \u00a0In this example, you see that there&#8217;s not much change in the difference between low values and high values, but these assessments can let you know if there&#8217;s an increasing tendency to under- or over-read at high and low values. In this way the &#8216;limits of agreement&#8217; plots tell you far more, far more meaningfully, that a scatter plot ever would.<\/p>\n<p>(There&#8217;s a great lecture\/webpage of a lecture <a href=\"http:\/\/www-users.york.ac.uk\/~mb55\/talks\/oxtalk.htm\">here<\/a> &#8211; on the maths and more behind the plots. If you&#8217;ve ever heard Prof Bland speak it&#8217;s written in his tone &amp; metre &#8230;)<\/p>\n<p>&#8211; Archi<!--TrendMD v2.4.8--><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Measuring things is what we do lots of, and we often want to measure things with a new machine. New, faster, shinier, cheaper, less invasive or more colourful &#8230; but we are almost always sold it as being highly correlated with the reference standard (p&lt;0.001). Think &#8211; what is this correlation and p-value telling us? [&#8230;]<\/p>\n<p><a class=\"btn btn-secondary understrap-read-more-link\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/09\/27\/statsminiblog-bland-altman-plots\/\">Read More&#8230;<\/a><\/p>\n","protected":false},"author":7,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2676],"tags":[],"class_list":["post-385","post","type-post","status-publish","format-standard","hentry","category-stats"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>StatsMiniBlog: Bland Altman Plots - ADC Online Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/09\/27\/statsminiblog-bland-altman-plots\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"StatsMiniBlog: Bland Altman Plots - ADC Online Blog\" \/>\n<meta property=\"og:description\" content=\"Measuring things is what we do lots of, and we often want to measure things with a new machine. 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