A CosmoMC convergence issue

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
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Erick Almaraz
Posts: 25
Joined: June 10 2012
Affiliation: University of Mexico

A CosmoMC convergence issue

Post by Erick Almaraz » June 16 2016

Hi everybody.

I have been running a CosmoMC analysis to find the constraints for a model of dynamical dark energy. After three weeks (the number of samples has surpassed 200 K - two chains), the R-1 values stored in the log files have been wandering around 0.01< R-1< 0.02, the plots retrieved from GetDist GUI show barely noticeable changes from time to time and still the program is running because it doesn't fulfill the R-1 < 0.01 stopping criterion. On the other hand, if I use the GetDist tools to assess the convergence (particularly, "getConvergeTests" ), I get
var(mean)/mean(var), remaining chains, worst e-value: R-1 = 0.00824
According to the "CosmoMC readme", there are differences between the GetDist and MPI run-time statistics, so I suppose that this discrepancy is normal. However, I don't know, given the amount of samples obtained and the results from GetDist, if it is safety to stop the program and take the results as reliable constraints to the model I have been working with.

Any idea?

Antony Lewis
Posts: 1369
Joined: September 23 2004
Affiliation: University of Sussex
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Re: A CosmoMC convergence issue

Post by Antony Lewis » June 16 2016

You probably have a broad tailed distribution, and/or weak multi-modality, which can make convergence slow. R-1 =0.01 is very good convergence, so 0.02 is probably fine, though look at 2d plots to understand if the shape looks reasonable.

Erick Almaraz
Posts: 25
Joined: June 10 2012
Affiliation: University of Mexico

A CosmoMC convergence issue

Post by Erick Almaraz » June 16 2016

Thanks for the quick reply!

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