On Twitter, I was asked if there were serious research papers published on coffee consumption and labour productivity. There are some papers on coffee breaks and productivity, e.g. Productivity Through Coffee Breaks, but I could not find anything on coffee consumptions. Since I could not find any dataset with personal consumption (maybe I should start keeping tracks of my own consumption to run a study) I tried to find data for national consumption instead (even if we know that both are – clearly – not equivalent)

• last year, Sabine published on http://backreaction.blogspot.fr/ a dataset with consumption of coffee, per country (and per unhabitants),
• on http://en.wikipedia.org/ we can find a dataset with GDP per hour worked for some countries (which can be seen as a common  measure of the productivity of a country)

If we merge those two datasets, we get

> base=read.table(
+ "http://freakonometrics.free.fr/cafe.csv",
+ header=TRUE,sep=";",dec=",")
> b=base[!is.na(base$GDP.PPP),]
> plot(b[,3],b[,4],xlab="Coffee Consumption",
+ ylab="GDP per hour worked")
> text(b[,3],b[,4]+1.6,b[,1],cex=.6)
> library(splines)
> X=b[,3]
> Y=b[,4]
> B=data.frame(X,Y)
> reg=glm(Y~bs(X),data=B)
> y=predict(reg,newdata=data.frame(
+ X=seq(0,10,by=.1)))
> lines(seq(0,10,by=.1),y,col="red")