What is the "default prior" in Stan ? Is it an "ideal" flat prior?

Dear STANers,

I was fitting a simple example with the model below and I am wondering what is the “default prior” which is used in this case.

I would think that it is a completely uniform/flat prior. Is that the case ?

data {
  int<lower=0> N;
  vector[N] x;
  vector[N] y;
parameters {
  real alpha;
  real beta;
  real<lower=0> sigma;
model {
    for (n in 1:N) {
      y[n] ~ normal(alpha + beta * x[n], sigma);



Yes, if you omit an explicit prior for a parameter it implies a uniform prior across that parameter’s domain.


Thank you for the clarification, Mike !

Cheers, Jozsef