Publishable Stuff

Rasmus Bååth's Research Blog

Posterior Update of Bayes@Lund 2016

For the third year round I and Ullrika Sahlin arranged Bayes@Lund, a mini-conference bringing together researchers interested in or working with Bayesian methods in and around Sweden. This year we were thrilled to have over 70 attendees, both from near and far, perhaps due to our interesting invited speakers Eric-Jan Wagenmakers and Robert Grant, or perhaps due to the promise of fika (a Swedish word referring to a break involving coffee and/or tea with cake and/or cookies and/or pastries, the more and the better). Perhaps it was a combination…

bayes.js: A Small Library for Doing MCMC in the Browser

Bayesian data analysis is cool, Markov chain Monte Carlo is the cool technique that makes Bayesian data analysis possible, and wouldn’t it be coolness if you could do all of this in the browser? That was what I thought, at least, and I’ve now made bayes.js: A small JavaScript library that implements an adaptive MCMC sampler and a couple of probability distributions, and that makes it relatively easy to implement simple Bayesian models in JavaScript.

Here is a motivating example: Say that you have the heights of the last ten American presidents…

// The heights of the last ten American presidents in cm, from Kennedy to Obama 
var heights = [183, 192, 182, 183, 177, 185, 188, 188, 182, 185];

… and that you would like to fit a Bayesian model assuming a Normal distribution to this data. Well, you can do that right now by clicking “Start sampling” below! This will run an MCMC sampler in your browser implemented in JavaScript.

If this doesn’t seem to work in your browser, for some reason, then try this version of the demo.

A Bayesian Model to Calculate Whether My Wife is Pregnant or Not

On the 21st of February, 2015, my wife had not had her period for 33 days, and as we were trying to conceive, this was good news! An average period is around a month, and if you are a couple trying to go triple, then a missing period is a good sign something is going on. But at 33 days, this was not yet a missing period, just a late one, so how good news was it? Pretty good, really good, or just meh?

To get at this I developed a simple Bayesian model that, given the number of days since your last period and your history of period onsets, calculates the probability that you are going to be pregnant this period cycle. In this post I will describe what data I used, the priors I used, the model assumptions, and how to fit it in R using importance sampling. And finally I show you why the result of the model really didn’t matter in the end. Also I’ll give you a handy script if you want to calculate this for yourself. :)

The Map of Romantic Kissing with Leaflet and R

Romantic kissing is a cultural universal, right? Nope! At least not if you are to believe Jankowiak et al. (2015) who surveyed a large number of cultures and found that “sexual-romantic kissing” occurred in far from all of them. For some reasons the paper didn’t include a world map with these kissers and non-kissers plotted out. So, with the help of my colleague Andrey Anikin I’ve now made such a map using R and the excellent leaflet package. Click on the image below to check it out:

Basic MCMC and Bayesian statistics in… BASIC!

The BASIC programming language was at one point the most widely spread programming language. Many home computers in the 80s came with BASIC (like the Commodore 64 and the Apple II), and in the 90s both DOS and Windows 95 included a copy of the QBasic IDE. QBasic was also the first programming language I encountered (I used it to write a couple of really horrible text adventures). Now I haven’t programmed in BASIC for almost 20 years and I thought I would revisit this (from my current perspective) really weird language. As I spend a lot of time doing Bayesian data analysis, I though it would be interesting to see what a Bayesian analysis would look like if I only used the tool that I had 20 years ago, that is, BASIC.

This post walks through the implementation of the Metropolis-Hastings algorithm, a standard Markov chain Monte Carlo (MCMC) method that can be used to fit Bayesian models, in BASIC. I then use that to fit a Laplace distribution to the most adorable dataset that I could find: The number of wolf pups per den from a sample of 16 wold dens. Finally I summarize and plot the result, still using BASIC. So, the target audience of this post is the intersection of people that have programmed in BASIC and are into Bayesian computation. I’m sure you are out there. Let’s go!

Easy Bayesian Bootstrap in R

A while back I wrote about how the classical non-parametric bootstrap can be seen as a special case of the Bayesian bootstrap. Well, one difference between the two methods is that, while it is straightforward to roll a classical bootstrap in R, there is no easy way to do a Bayesian bootstrap. This post, in an attempt to change that, introduces a bayes_boot function that should make it pretty easy to do the Bayesian bootstrap for any statistic in R. If you just want a function you can copy-n-paste into R go to The bayes_boot function below. Otherwise here is a quick example of how to use the function, followed by some details on the implementation.

Update: I’ve now created an R package that implements the Bayesian bootstrap, which I recommend instead of using the function described in this post. You can install it by running install.packages("bayesboot") in R and you can read more about it here. The implementation is the same as here, but the interface is slightly different.

Hygge at UseR! 2015, Aalborg

A Danish word (pronounced HU-guh) meaning social coziness. I.e. the feeling of a good social atmosphere.Urban Dictionary

Yes, there was plenty of hygge to go around this year’s UseR! that took place last week in Aalborg, Denmark. Everybody I’ve spoken with agrees that it was an extraordinary conference, from the interesting speakers and presentations to the flawless organization (spearheaded by Torben Tvedebrink) and the warm weather. As there were many parallel session, I only managed to attend a fraction of the talks, but here are some of my highlights:

Big Data and Chess Follow-up: Predictive Piece Values Over the Course of a Game

In a previous post I used the the Million Base 2.2 chess data base to calculate the predictive piece values of chess pieces. It worked out pretty well and here, just for fun, I thought I would check out what happens with the predictive piece values over the course of a chess game. In the previous analysis, the data (1,000,000 chess positions) was from all parts of the chess games. Here, instead, are the predictive piece values using only positions up to the 10th first full moves (a full move is when White and Black each have made a move):

Big Data and Chess: What are the Predictive Point Values of Chess Pieces?

Who doesn’t like chess? Me! Sure, I like the idea of chess – intellectual masterminds battling each other using nothing but pure thought – the problem is that I tend to lose, probably because I don’t really know how to play well, and because I never practice. I do know one thing: How much the different pieces are worth, their point values:

This was among the first things I learned when I was taught chess by my father. Given these point values it should be as valuable to have a knight on the board as having three pawns, for example. So where do these values come from? The point values are not actually part of the rules for chess, but are rather just used as a guideline when trading pieces, and they seem to be based on the expert judgment of chess authorities. (According to the guardian of truth there are many alternative valuations, all in the same ballpark as above.) As I recently learned that it is very important to be able to write Big Data on your CV, I decided to see if these point values could be retrieved using zero expert judgement in favor of huge amounts of chess game data.