This is a dummy sampler that just evaluates the likelihood at a reference point. You can use it to test your likelihoods (take a look too at the model wrapper for a similar but more interactive tool).
To use it, simply make the
sampler: evaluate: # Optional: override parameter values override: # param: value
The posterior will be evaluated at a point sampled from the reference pdf (which may be a fixed value) or from the prior if there is no reference. Values passed through
evaluate:override will take precedence. For example:
params: a: prior: min: -1 max: 1 ref: 0.5 b: prior: min: -1 max: 1 ref: dist: norm loc: 0 scale: 0.1 c: prior: min: -1 max: 1 d: prior: min: -1 max: 1 ref: 0.4 sampler: evaluate: override: d: 0.2
In this case, the posterior will be evaluated for each parameter at:
a: Exactly at \(0.5\).
b: Sampled from the reference pdf: a Gaussian centred at \(0\) with standard deviation \(0.1\).
c: From the prior, since there is no reference pdf: sampled uniformly in the interval \([-1, 1]\).
d: From the
override, which takes precedence above all else.
If using this sampler cobaya appears to be stuck, this normally means that it cannot sample a point with finite posterior value. Check that your prior/likelihood definitions leave room for some finite posterior density, e.g. don’t define an external prior that imposes that \(x>2\) if the range allowed for \(x\) is just \([0,1]\).