Uncommon Ground

Monthly Archive: September 2016

Being Bayesian won’t save you

Last week I pointed out that you should

Be wary of results from studies with small sample sizes, even if the effects are statistically significant.

Now you may be thinking to yourself: “I’m a Bayesian, and I use somewhat informative priors. This doesn’t apply to me.” Well, I’m afraid you’re wrong. Here are results from analysis of data simulated according to the same conditions I used last week in exploring P-values. The prior on each mean is N(0, 1), and the prior on each standard deviation is half-N(0, 1).

Mean Sample size Power Wrong sign
0.05 10 39/1000 18/39
50 59/1000 12/59
100 47/1000 5/47
0.10 10 34/1000 8/34
50 81/1000 10/81
100 115/1000 6/115
0.20 10 62/1000 7/62
50 158/1000 2/158
100 292/1000 0/292

Here “Power” refers to the number of times (out of 1000 replicates) the symmetric 95% credible intervals do not overlap 0, which is when we’d normally conclude we have evidence that the means of the two populations are different. Notice that when the effect and sample size are small (0.05 and 10, respectively), we would infer the wrong sign for the difference almost half of the time (18/39). We’re less likely to make a sign error when the effect is larger (7/62 for an effect of 0.20) or when the sample size is large (5/47 for a sample size of 100). But the bottom line remains the same:

Be wary of results from studies with small sample sizes, even if the effects are statistically significant.

This figure summarizes results from the simulation, and you’ll find the code in the same Github repository as the P-value code I mentioned last week: https://github.com/kholsinger/noisy-data. Remember that Gelman and Carlin (Perspectives on Psychological Science 9:641; 2014 http://dx.doi.org/10.1177/1745691614551642)  also have advice on how to tell whether you’re data are too noisy for your sample to give confidence in your inferences.


Thought for the day

If a man walk in the woods for love of them half of each day, he is in danger of being regarded as a loafer; but if he spends his whole day as a speculator, shearing off those woods and making earth bald before her time, he is esteemed an industrious and enterprising citizen.

Henry David Thoreau, Life without principle

Edwin Way Teale Series on Nature and the Environment

Teale 2016Every year since the 1997 the University of Connecticut has hosted the Edwin Way Teale Lecture Series on Nature and the Environment. The series features distinguished natural scientists, social scientists, authors, artists, performers, and policy makers whose work informs our understanding of nature and the environment. The lectures are free and open to the public. Many lectures in recent years are also available online. You can find the full list of past lectures and links to videos (where available) at this link: http://lib.uconn.edu/about/events/nature-the-environment-the-edwin-way-teale-lecture-series-past-lectures/.

Here is a quick list of this year’s events:

  • Julien Agyeman, “Just Sustainabilities: Re-imagining e/quality, Living Within Limits”
  • Emma Rosi-Marshall, “Our Rivers on Drugs: Pharmaceuticals and Personal Care Products as Agents of Ecological Change in Aquatic Ecosystems”
  • Harriet Ritvo, “Wanting the Wild”
  • Elizabeth Kolbert, “The Sixth Extinction”
  • Maria Carmen Lemos, “Building Capacity for Adapting to Climate Change”
  • Mina Girgis, “The Nile Project”

The dates and times for the events are available on the Teale Series website. If you are close to Storrs, please stop by and join us. If you are far away or other commitments mean that you can’t join us, please check back to see if a recorded version of the presentation that interests you is available online.

Legos and graduate school

Screen Shot 2016-09-01 at 12.49.36 PMGraduate students are very creative, and I recently learned about an anonymous graduate student in her/his sixth year at a private, West Coast university who is more creative than most – @legogradstudent. I’ve been out of graduate school for more years than I like to admit,1 but I can still relate to the feelings @legogradstudent captures in her/his tweets. S/he has just short of 2600 followers now, but I’m sure that number is going to grow. Inside Higher Ed described her/him this way in the article that brought her/him to my attention:

Lego Grad Student has fans across disciplines, who often use some variation of “devastatingly true” to describe his experiences. Indeed, his tableaux focus not on the intricacies of his research but rather on the human experience of graduate school: feelings of being on a treadmill to nowhere, being beaten to the intellectual punch by colleagues, using sophisticated avoidance techniques during a class discussion and the horror of seeing free food disappear before his eyes at departmental events.

If you’re in graduate school, if you have friends or relatives who are in graduate school, or if you’re just interested in graduate school, you owe it to yourself to follow @legogradstudent on Twitter or Instagram.

134 years last June, if you must know.