# What is the difference between sample algorithms and algoritms for estimating parameters?

#### luchins

##### Member
I have red this in the website:

• MCMC is a family of sampling algorithms, which means given a distribution, these algorithms return samples according to this distribution. Many problem, bayesian posterior inference for instance, require you compute the posterior distribution P(θ|D), most of the time has no close form solution, so instead of get the actually form of P(θ|D), you sample from it, after you collect the samples, you can use these samples to estimate θ. So, MCMC is a sampling algorithm, not a algorithm for estimating paramters.

What is the difference between a sampling algoritms and algorithma for estimating paramters?

#### spunky

##### Doesn't actually exist
Well, you can still use MCMC to estimate parameters by doing something on the posterior distribution after you've run all your chains. On the (I'll admit, limited) work I did on this for my MA thesis, I used a type of MCMC (Gibbs sampler) to get a posterior distribution and then I calculated the MAP (Maximum A Posteriori) on that distribution to get a parameter estimate. I guess that's as opposed to something like Maximum Likelihood where after optimizing the likelihood function, you get a parameter estimate as a solution. So MCMCs by themselves won't give you a parameter estimate, but you can take one more step to get one.

#### hlsmith

##### Not a robit
Yes, it can be interpreted as an algorithm to get (simulate) samples used by another algorithm that calculate estimates based on those samples, which then results in a posterior distribution consisting of a distribution of estimates. So say you simulate 1,000 samples, then for each sample you run the estimate algorithm on it, so you end up with 1,000 estimates.