As the normal distribution is sort of the default choice when modeling continuous data (but not necessarily the best choice), the Poisson distribution is the default when modeling counts of events. Indeed, when all you know is the number of events during a certain period it is hard to think of any other distribution, whether you are modeling the number of deaths in the Prussian army due to horse kicks or the numer of goals scored in a football game. Like the t.test
in R there is also a poisson.test
that takes one or two samples of counts and spits out a pvalue. But what if you have some counts, but don’t significantly feel like testing a null hypothesis? Stay tuned!
Subjective Rhythmization at ICMPC 2014 in Seoul
I was lucky enough to be presenting at the 14th International Conference on Music Perception and Cognition in Seoul, South Korea, last week. It was a very inspiring conference and I really like South Korea (especially the amazing food…). My presentation was on the topic of subjective rhythmization, a fascinating auditory illusion. See below for my presentation slides and for a short conference paper that was published in the proceedings of the conference:
Bååth, R., Ingvarsdóttir, K. O. (2014) Subjective Rhythmization: A Replication And an Extension. Proceedings of the 13th International Conference on Music Perception and Cognition in Seoul, South Korea. pdf of full paper
drinkR: Estimate your Blood Alcohol Concentration using R and Shiny.
Inspired by events that took place at UseR 2014 last month I decided to implement an app that estimates one’s blood alcohol concentration (BAC). Today I present to you drinkR, implemented using R and Shiny, Rstudio’s framework for building web apps using R. So, say that I had a good dinner, drinking a couple of glasses of wine, followed by an evening at a divy karaoke bar, drinking a couple of red needles and a couple of beers. By entering my sex, height and weight and the times when I drank the drinks in the drinkR app I end up with this estimated BAC curve:
(Now I might be totally off with what drinks I had and when but Romain Francois, Karl Broman, Sandy Griffith, Karthik Ram and Hilary Parker can probably fill in the details.) If you want to estimate your current BAC (or a friend’s…) then head over to the drinkr app hosted at ShinyApps.io. If you want to know how how the app estimates BAC read on below. The code for drinkR is available on GitHub, any suggestion on how it can be improved is greatly appreciated.
Chillin’ at UseR! 2014
This year’s UseR! conference was held at the University of California in Los Angeles. Despite the great weather and a nearby beach, most of the conference was spent in front of projector screens in 18° c (64° f) rooms because there were so many interesting presentations and tutorials going on. I was lucky to present my R package Bayesian First Aid and the slides can be found here:
beepr (former pingr) is on CRAN. It’s easier than ever to make R go beep!
Even though I said it would never happen, my silly package with the sole purpose of playing notification sounds is now on CRAN. Big thanks to the CRAN maintainers for their patience! For instant gratification run the following in R to install beepr
and make R produce a notification sound:
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Bayesian First Aid: Test of Proportions
Does pill A or pill B save the most lives? Which web design results in the most clicks? Which in vitro fertilization technique results in the largest number of happy babies? A lot of questions out there involves estimating the proportion or relative frequency of success of two or more groups (where success could be a saved life, a click on a link, or a happy baby) and there exists a little known R function that does just that, prop.test
. Here I’ll present the Bayesian First Aid version of this procedure. A word of caution, the example data I’ll use is mostly from the Journal of Human Reproduction and as such it might be slightly NSFW :)
The Most Comprehensive Review of Comic Books Teaching Statistics
As I’m more or less an autodidact when it comes to statistics, I have a weak spot for books that try to introduce statistics in an accessible and pedagogical way. I have therefore collected what I believe are all books that introduces statistics using comics (at least those written in English). What follows are highly subjective reviews of those four books. If you know of any other comic book on statistics, please do tell me!
I’ll start with a tl;dr version of the reviews, but first here are the four books:
Jeffreys’ Substitution Posterior for the Median: A Nice Trick to Nonparametrically Estimate the Median
While reading up on quantile regression I found a really nice hack described in Bayesian Quantile Regression Methods (Lancaster & Jae Jun, 2010). It is called Jeffreys’ substitution posterior for the median, first described by Harold Jeffreys in his Theory of Probability, and is a nonparametric method for approximating the posterior of the median. What makes it cool is that it is really easy to understand and pretty simple to compute, while making no assumptions about the underlying distribution of the data. The method does not strictly produce a posterior distribution, but has been shown to produce a conservative approximation to a valid posterior (Lavine, 1995). In this post I will try to explain Jeffreys’ substitution posterior, give Rcode that implements it and finally compare it with a classical nonparametric test, the Wilcoxon signedrank test. But first a picture of Sir Harold Jeffreys:
Bayesian First Aid: Pearson Correlation Test
Correlation does not imply causation, right but, as Edward Tufte writes, “it sure is a hint.” The Pearson productmoment correlation coefficient is perhaps one of the most common ways of looking for such hints and this post describes the Bayesian First Aid alternative to the classical Pearson correlation test. Except for being based on Bayesian estimation (a good thing in my book) this alternative is more robust to outliers and comes with a pretty nice default plot. :)
A Hack to Create Matrices in R, Matlab style
The Matlab syntax for creating matrices is pretty and convenient. Here is a 2x3 matrix in Matlab syntax where ,
marks a new column and ;
marks a new row:
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Here is how to create the corresponding matrix in R:
1


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Functional but not as pretty, plus the default is to specify the values column wise. A better solution is to use rbind
: