{"id":849,"date":"2023-01-28T15:52:22","date_gmt":"2023-01-28T15:52:22","guid":{"rendered":"https:\/\/www.allendowney.com\/blog\/?p=849"},"modified":"2023-01-28T15:52:22","modified_gmt":"2023-01-28T15:52:22","slug":"never-test-for-normality","status":"publish","type":"post","link":"https:\/\/www.allendowney.com\/blog\/2023\/01\/28\/never-test-for-normality\/","title":{"rendered":"Never Test for Normality"},"content":{"rendered":"\n<p>Way back in 2013, I wrote <a href=\"https:\/\/allendowney.blogspot.com\/2013\/08\/are-my-data-normal.html\">this blog post<\/a> explaining why you should never use a statistical test to check whether a sample came from a Gaussian distribution. I argued that data from the real world never come from a Gaussian distribution, or any other simple mathematical model, so the answer to the question is always no. And there are only two possible outcomes from the test:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>If you have enough data, the test will reject the hypothesis that the data came from a Gaussian distribution, or<\/li>\n\n\n\n<li>If you don\u2019t have enough data, the test will fail to reject the hypothesis.<\/li>\n<\/ul>\n\n\n\n<p>Either way, the result doesn\u2019t tell you anything useful.<\/p>\n\n\n\n<p>In this article, I will explore a particular example and demonstrate this relationship between the sample size and the outcome of the test. And I will conclude, again, that<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>Choosing a distribution is not a statistical question; it is a modeling decision. No statistical test can tell you whether a particular distribution is a good model for your data.<\/p>\n<\/blockquote>\n\n\n\n<p>For the technical details, you can <a href=\"https:\/\/allendowney.github.io\/ElementsOfDataScience\/anderson.html\">read the extended version of this article<\/a> or <a href=\"https:\/\/colab.research.google.com\/github\/AllenDowney\/ElementsOfDataScience\/blob\/master\/examples\/anderson.ipynb\">run this notebook on Colab<\/a>.<\/p>\n\n\n\n<p>I\u2019ll start by generating a sample that is actually from a lognormal distribution, then use the sample mean and standard deviation to make a Gaussian model. Here\u2019s what the empirical distribution of the sample looks like compared to the CDF of the Gaussian distribution. <\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"683\" src=\"https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-1024x683.png\" alt=\"\" class=\"wp-image-851\" srcset=\"https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-1024x683.png 1024w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-300x200.png 300w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-768x512.png 768w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-1536x1024.png 1536w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1-405x270.png 405w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson1-1.png 1800w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<p>It looks like the Gaussian distribution is a pretty good model for the data, and probably good enough for most purposes.<\/p>\n\n\n\n<p>According to the <a href=\"https:\/\/docs.scipy.org\/doc\/scipy\/reference\/generated\/scipy.stats.anderson.html\">Anderson-Darling test<\/a>, the test statistic is 1.7, which exceeds the critical value, 0.77, so at the 5% significance level, we can reject the hypothesis that this sample came from a Gaussian distribution. That\u2019s the right answer, so it might seem like we\u2019ve done something useful. But we haven\u2019t.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Sample size<\/h2>\n\n\n\n<p>The result from the A-D test depends on the sample size. The following figure shows the probability of rejecting the null hypothesis as a function of sample size, using the lognormal distribution from the previous section.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"683\" src=\"https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-1024x683.png\" alt=\"\" class=\"wp-image-852\" srcset=\"https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-1024x683.png 1024w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-300x200.png 300w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-768x512.png 768w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-1536x1024.png 1536w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2-405x270.png 405w, https:\/\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/01\/anderson2.png 1800w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/figure>\n\n\n\n<p>When the sample size is more than 200, the probability of rejection is high. When the sample size is less than 100, the probability of rejection is low. But notice that it doesn\u2019t go all the way to zero, because there is always a 5% chance of a false positive.<\/p>\n\n\n\n<p>The critical value is about 120; at that sample size, the probability of rejecting the null is close to 50%.<\/p>\n\n\n\n<p>So, again, if you have enough data, you\u2019ll reject the null; otherwise you probably won\u2019t. Either way, you learn nothing about the question you really care about, which is whether the Gaussian model is a <em>good enough<\/em> model of the data for your purposes.<\/p>\n\n\n\n<p>That\u2019s a modeling decision, and no statistical test can help. In <a href=\"https:\/\/allendowney.blogspot.com\/2013\/08\/are-my-data-normal.html\">the original article<\/a>, I suggested some methods that might.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Way back in 2013, I wrote this blog post explaining why you should never use a statistical test to check whether a sample came from a Gaussian distribution. I argued that data from the real world never come from a Gaussian distribution, or any other simple mathematical model, so the answer to the question is always no. And there are only two possible outcomes from the test: Either way, the result doesn\u2019t tell you anything useful. In this article, I&#8230;<\/p>\n<p class=\"read-more\"><a class=\"btn btn-default\" href=\"https:\/\/www.allendowney.com\/blog\/2023\/01\/28\/never-test-for-normality\/\"> Read More<span class=\"screen-reader-text\">  Read More<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1],"tags":[],"class_list":["post-849","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Never Test for Normality - Probably Overthinking It<\/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:\/\/www.allendowney.com\/blog\/2023\/01\/28\/never-test-for-normality\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Never Test for Normality - Probably Overthinking It\" \/>\n<meta property=\"og:description\" content=\"Way back in 2013, I wrote this blog post explaining why you should never use a statistical test to check whether a sample came from a Gaussian distribution. 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If you are comparing an empirical distribution to a model, the CDF gives you the best view of any differences between the data and the model. Now I want to amend my advice. CDFs give you a good\u2026","rel":"","context":"In \"CDF\"","block_context":{"text":"CDF","link":"https:\/\/www.allendowney.com\/blog\/tag\/cdf\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2019\/08\/image-16.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":703,"url":"https:\/\/www.allendowney.com\/blog\/2022\/05\/02\/how-gaussian-is-it\/","url_meta":{"origin":849,"position":1},"title":"How Gaussian Is It?","author":"AllenDowney","date":"May 2, 2022","format":false,"excerpt":"This article is an excerpt from the current draft of my book Probably Overthinking It, to be published by the University of Chicago Press in early 2023. If you would like to receive infrequent notifications about the book (and possibly a discount), please sign up for this mailing list.This book\u2026","rel":"","context":"In \"Gaussian\"","block_context":{"text":"Gaussian","link":"https:\/\/www.allendowney.com\/blog\/tag\/gaussian\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2022\/04\/ansur_30_0.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1109,"url":"https:\/\/www.allendowney.com\/blog\/2023\/11\/21\/life-in-a-lognormal-world\/","url_meta":{"origin":849,"position":2},"title":"Life in a Lognormal World","author":"AllenDowney","date":"November 21, 2023","format":false,"excerpt":"At PyData Global 2023 I will present a talk, \"Extremes, outliers, and GOATs: On life in a lognormal world\". It is scheduled for Wednesday 6 December at 11 am Eastern Time. [UPDATE: here is the video] https:\/\/youtu.be\/MhA5XWIWWys Here is the abstract: The fastest runners are much faster than we expect\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=700%2C400&ssl=1 2x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=1050%2C600&ssl=1 3x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/11\/log_vs_normal5-1.png?resize=1400%2C800&ssl=1 4x"},"classes":[]},{"id":1646,"url":"https:\/\/www.allendowney.com\/blog\/2025\/11\/04\/its-levels\/","url_meta":{"origin":849,"position":3},"title":"It&#8217;s Levels","author":"AllenDowney","date":"November 4, 2025","format":false,"excerpt":"A recent Reddit post asks \"Amateur athletes of Reddit: what's your 'There's levels to this shit' experience from your sport?\" Responses included: We have some good runners who can win local races ... And then you realise that if you put them in a 5000m race with Olympic-level athletes they'd\u2026","rel":"","context":"In \"lognormal distribution\"","block_context":{"text":"lognormal distribution","link":"https:\/\/www.allendowney.com\/blog\/tag\/lognormal-distribution\/"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2025\/11\/212090bf9c6718b7d51f02945d3cb353d999e9243739e5c24535e8f5f36f013e.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1085,"url":"https:\/\/www.allendowney.com\/blog\/2023\/10\/28\/why-are-you-so-slow\/","url_meta":{"origin":849,"position":4},"title":"Why are you so slow?","author":"AllenDowney","date":"October 28, 2023","format":false,"excerpt":"Recently a shoe store in France ran a promotion called \"Rob It to Get It\", which invited customers to try to steal something by grabbing it and running out of the store. But there was a catch -- the \"security guard\" was a professional sprinter, M\u00e9ba Mickael Zeze. As you\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/10\/running2.png?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/10\/running2.png?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/10\/running2.png?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/www.allendowney.com\/blog\/wp-content\/uploads\/2023\/10\/running2.png?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":1210,"url":"https:\/\/www.allendowney.com\/blog\/2024\/02\/01\/extremes-outliers-and-goats\/","url_meta":{"origin":849,"position":5},"title":"Extremes, outliers, and GOATS","author":"AllenDowney","date":"February 1, 2024","format":false,"excerpt":"The video from my PyData Global 2023 talk, Extremes, outliers, and GOATS, is available now: https:\/\/www.youtube.com\/watch?v=MhA5XWIWWys The slides are here. There are two Jupyter notebooks that contain the analysis I presented: Chapter 4 of Probably Overthinking It An exploration of Gaussian and lognormal models for anthropomorphic measurements Here's the abstract:\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/img.youtube.com\/vi\/MhA5XWIWWys\/0.jpg?resize=350%2C200","width":350,"height":200},"classes":[]}],"_links":{"self":[{"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/posts\/849","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/comments?post=849"}],"version-history":[{"count":1,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/posts\/849\/revisions"}],"predecessor-version":[{"id":853,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/posts\/849\/revisions\/853"}],"wp:attachment":[{"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/media?parent=849"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/categories?post=849"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.allendowney.com\/blog\/wp-json\/wp\/v2\/tags?post=849"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}