{"id":3161,"date":"2008-11-17T08:58:38","date_gmt":"2008-11-17T08:58:38","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2008\/11\/17\/think-before-you-plot\/"},"modified":"2008-11-17T08:58:38","modified_gmt":"2008-11-17T08:58:38","slug":"think-before-you-plot","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2008\/11\/17\/think-before-you-plot\/","title":{"rendered":"Think Before You Plot"},"content":{"rendered":"

There’s a link in today’s links dump to a post from Pictures of Numbers<\/a>, a rarely-updated blog on the visual presentation of data (via Swans On Tea<\/a>, I think). There’s some really good stuff there about how to make graphs that are easy to read and interpret.<\/p>\n

I would like to dissent mildly from one of their points, in the Better Axes<\/a> post, specifically the advice about not starting at zero. In many cases, this is good advice, but like most rules of thumb, it shouldn’t be followed too closely.<\/p>\n

Take, for example, this post from one of my metastable xenon papers<\/a>:<\/p>\n

\"i-78c4a722dd0e280b15c0ae6cc2e1cfdd-fig3.jpg\"<\/p>\n

<\/p>\n

A strict application of the presentation rules promoted by people like Edward Tufte (which are generally quite similar to the things Pictures of Numbers says) would say that there’s too much white space in this graph, particularly at the bottom. The variation in the data would be much more obvious if the vertical axis started at 0.5 instead of 0.<\/p>\n

The problem is, that would be a deceptive presentation of the data. The whole point of this graph, in the context of the research, is that there’s actually very little variation in the data. The points show the relative collision rates for various different isotopes of xenon, and they’re all more or less the same. Blowing up the axis would make what differences there are more obvious, but that would distort the point of the graph.<\/p>\n

This comes up a lot in intro labs, in which we frequently ask students to make null measurements– to measure how the period of a pendulum depends on the mass, for example, or how the period of a mass oscillating on a spring depends on the amplitude. Students will carefully measure a half-dozen points, pop them into Excel, and make a graph that is auto-scaled so that the data points span the full vertical range of the plot. Then they’ll fit a trend line to the data, and declare that the period increases linearly with the mass. And if you look at the plot, it looks for all the world as if the trend line slopes impressively from one corner to the other.<\/p>\n

Of course, the period of a pendulum doesn’t depend on the mass, so what they’re fitting is just noise. The dramatic sloping trend lines are dramatic only because the scale is blown up so much. If the vertical scale went all the way to zero, it would be clear that there’s no variation worth mentioning.<\/p>\n

I’m not saying that you should totally abandon the principles Tufte and Pictures of Numbers suggest– for most cases, their advice is good advice. It’s important to remember that all these rules have exceptions, though, and to think carefully about what you’re trying to show before you plot your data.<\/p>\n","protected":false},"excerpt":{"rendered":"

There’s a link in today’s links dump to a post from Pictures of Numbers, a rarely-updated blog on the visual presentation of data (via Swans On Tea, I think). There’s some really good stuff there about how to make graphs that are easy to read and interpret. I would like to dissent mildly from one… Continue reading Think Before You Plot<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8,13,19,9,7,11],"tags":[],"class_list":["post-3161","post","type-post","status-publish","format-standard","hentry","category-academia","category-education","category-experiment","category-math","category-physics","category-science","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3161","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=3161"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3161\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=3161"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=3161"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=3161"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}