{"id":792,"date":"2013-12-15T20:21:24","date_gmt":"2013-12-15T19:21:24","guid":{"rendered":"https:\/\/blogs.bmj.com\/adc\/?p=792"},"modified":"2013-12-02T10:51:38","modified_gmt":"2013-12-02T09:51:38","slug":"basics-size-vs-bias","status":"publish","type":"post","link":"https:\/\/blogs.bmj.com\/adc\/2013\/12\/15\/basics-size-vs-bias\/","title":{"rendered":"Basics. Size vs. bias"},"content":{"rendered":"<p>There&#8217;s a beautifully clear <a href=\"http:\/\/ebm.bmj.com\/content\/early\/2013\/11\/26\/eb-2013-101610.full\">explanation behind the BMJ-EBM-journal paywall <\/a>of a concept I&#8217;ve been struggling to express for some time, which is partly there in <a title=\"GRADE it.\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/04\/03\/grade-it\/\">GRADE<\/a>\u00a0and partly grounded in common sense.<\/p>\n<p>Take the <a href=\"http:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC300808\/\">parachute argument<\/a> &#8212; do you really need an RCT for parachutes (as there are survivors of non-&#8216;chuted \u00a0falls) &#8212; and reductio ad absurdium leaps to &#8216;so all EBM is bunk&#8217;. As<a title=\"Where does EBM go without E?\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/08\/28\/where-does-ebm-go-without-e\/\"> discussed earlier<\/a>, EBM is not all RCT so the particular strawman here fires brightly away from anything meaningful, but is does illuminate a problem. What about situations where non-RCT evidence is good enough?<\/p>\n<p>The &#8216;<a href=\"http:\/\/www.ncbi.nlm.nih.gov\/pmc\/articles\/PMC3503923\/\">mothers kiss<\/a>&#8216; for nostrilly based crayons is a good example. It works; so why do an RCT? Well, it&#8217;s not just that &#8211; it works, and it&#8217;s unlikely to cause harm, and it&#8217;s a situation where the crayon&#8217;s not coming out on its own &#8211; so why do an RCT?<\/p>\n<p>To frame it alternatively, &#8220;what biases would have to be present in these observational studies, and how large would these biases have to be, in order to invalidate the result?&#8221; If the answer to this is SO large you wouldn&#8217;t believe it was possible, then you don&#8217;t need an RCT. The smaller the proposed effect size, the greater the need for randomised trial data. As a rule of thumb, if the effect is a relative risk of &gt;5 (or &lt;0.2 aka &lt; 1\/5th) \u00a0then you&#8217;ll be happy with good observational data. The closer it gets to &gt;2 \/ &lt;0.5, the more and more possible a biased explanation of the result is.<\/p>\n<p>To shorthand &#8211; appraise your evidence (i.e. assess the size of the threats to validity), evaluate the importance of the effect, and ask how closely it fits your PICO &#8211; don&#8217;t go all SR = 1a and CaseStudy = 4 on us.<\/p>\n<p>&#8211; Archi<!--TrendMD v2.4.8--><\/p>\n","protected":false},"excerpt":{"rendered":"<p>There&#8217;s a beautifully clear explanation behind the BMJ-EBM-journal paywall of a concept I&#8217;ve been struggling to express for some time, which is partly there in GRADE\u00a0and partly grounded in common sense. Take the parachute argument &#8212; do you really need an RCT for parachutes (as there are survivors of non-&#8216;chuted \u00a0falls) &#8212; and reductio ad [&#8230;]<\/p>\n<p><a class=\"btn btn-secondary understrap-read-more-link\" href=\"https:\/\/blogs.bmj.com\/adc\/2013\/12\/15\/basics-size-vs-bias\/\">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":[79],"tags":[],"class_list":["post-792","post","type-post","status-publish","format-standard","hentry","category-archimedes"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Basics. Size vs. bias - 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\/12\/15\/basics-size-vs-bias\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Basics. Size vs. bias - ADC Online Blog\" \/>\n<meta property=\"og:description\" content=\"There&#8217;s a beautifully clear explanation behind the BMJ-EBM-journal paywall of a concept I&#8217;ve been struggling to express for some time, which is partly there in GRADE\u00a0and partly grounded in common sense. Take the parachute argument &#8212; do you really need an RCT for parachutes (as there are survivors of non-&#8216;chuted \u00a0falls) &#8212; and reductio ad [...]Read More...\" \/>\n<meta property=\"og:url\" content=\"https:\/\/blogs.bmj.com\/adc\/2013\/12\/15\/basics-size-vs-bias\/\" \/>\n<meta property=\"og:site_name\" content=\"ADC Online Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-12-15T19:21:24+00:00\" \/>\n<meta name=\"author\" content=\"Bob Phillips\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Bob Phillips\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/blogs.bmj.com\\\/adc\\\/2013\\\/12\\\/15\\\/basics-size-vs-bias\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/blogs.bmj.com\\\/adc\\\/2013\\\/12\\\/15\\\/basics-size-vs-bias\\\/\"},\"author\":{\"name\":\"Bob Phillips\",\"@id\":\"https:\\\/\\\/blogs.bmj.com\\\/adc\\\/#\\\/schema\\\/person\\\/9e94029681ecf36e73bbd1eb2be2ef94\"},\"headline\":\"Basics. 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