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This afternoon, while I was discussing with Montserrat (aka @mguillen_estany) we were wondering how long it might take to run a regression model. More specifically, how long it might take if we use a Bayesian approach. My guess was that the time should probably be linear in , the number of observations. But I thought I would be good to check.

Let us generate a big dataset, with one million rows,

> n=1e6 > X=runif(n) > Y=2+5*X+rnorm(n) > B=data.frame(X,Y)

Consider as a benchmark the standard linear regression,

> lm_freq = function(n){ + idx = sample(1:1e6,size=n) + reg = lm(Y~X,data=B[idx,]) + summary(reg) + }

Here the regression is a subset of smaller size. We can do the same with a Bayesian approach, using stan,

> stan_lm =" + data { + int N; + vector[N] x; + vector[N] y; + } + parameters { + real alpha; + real beta; + real tau; + } + transformed parameters { + real sigma; + sigma <- 1 / sqrt(tau); + } + model{ + y ~ normal(alpha + beta * x, sigma); + alpha ~ normal(0, 10); + beta ~ normal(0, 10); + tau ~ gamma(0.001, 0.001); + } + "

Define then the model

> library(rstan) > system.time( stanmodel <<- stan_model(model_code = stan_lm)) utilisateur système écoulé 0.043 0.000 0.043

We want to see how long it might take to run a regression,

> lm_bayes = function(n){ + idx = sample(1:1e6,size=n) + fit = sampling(stanmodel, + data = list(N=n, + x=X[idx], + y=Y[idx]), + iter = 1000, warmup=200) + summary(fit) + }

We use the following package to see how long it takes

> library(microbenchmark) > time_lm = function(n){ + M = microbenchmark(lm_freq(n), + lm_bayes(n),times=50) + return(apply( matrix(M$time,nrow=2),1,mean)) + }

We can now compare the time it took with ten, one hundred, on thousand, and ten thousand observations,

> vN = c(10,100,1000,10000) > T = Vectorize(time_lm)(vN)

we can then plot it

> plot(vN,T[2,]/1e6,log="xy",col="red",type="b", + xlab="Number of Observations",ylab="Time") > lines(vN,T[1,]/1e6,col="blue",type="b")

It looks like (if we forget about the very small sample) that the time it takes to run a regression is linear, with the two techniques (the frequentist and the Bayesian ones).

And actually, the same story olds for logistic regressions. Consider the following dataset

> n=1e6 > X=runif(n) > S=-3+2*X+rnorm(n) > Y=rbinom(n,size=1,prob=exp(S)/(1+exp(S))) > B=data.frame(X,Y)

The frequentist version of the logistic regression is

> glm_freq = function(n){ + idx = sample(1:1e6,size=n) + reg = glm(Y~X,data=B[idx,],family=binomial) + summary(reg) + }

and the Bayesian one, using stan,

> stan_glm = " + data { + int N; + vector[N] x; + inty[N]; + } + parameters { + real alpha; + real beta; + } + model { + alpha ~ normal(0, 10); + beta ~ normal(0, 10); + y ~ bernoulli_logit(alpha + beta * x); + } + " > stanmodel = stan_model(model_code = stan_glm) ) > glm_bayes = function(n){ + idx = sample(1:1e6,size=n) + fit = sampling(stanmodel, + data = list(N=n, + x = X[idx], + y = Y[idx]), + iter = 1000, warmup=200) + summary(fit) + }

Again, we can see how long it takes to run those regression models

> time_gl m= function(n){ + M = microbenchmark(glm_freq(n), + glm_bayes(n),times=50) + return(apply( matrix(M$time,nrow=2),1,mean)) + }

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