## Meaning of different chi-squares in COSMOMC

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Brajesh Gupt
Posts: 6
Joined: January 04 2015
Affiliation: Pennsylvania State University

### Meaning of different chi-squares in COSMOMC

Hi,

After running getdist, one usually gets the several different types of chi-square values. What do they mean? For example, I get (I show only the best fit values) following 5 types:

Code: Select all

     0.2579038E+02     \chi^2_&#123;\rm lensing&#125;
0.7639290E+03     \chi^2_&#123;\rm plik&#125;
0.1049659E+05     \chi^2_&#123;\rm lowTEB&#125;
0.2068837E+01     \chi^2_&#123;\rm prior&#125;
0.1128631E+05    \chi^2_&#123;\rm CMB&#125;

While comparing two models which one should I use?

Any insight will be very helpful.

Thanks,
Brajesh

Stéphane Ilic
Posts: 17
Joined: February 11 2014
Affiliation: IRAP Toulouse

### Meaning of different chi-squares in COSMOMC

Hello Brajesh,

Those are the different contributions to the total likelihood/chi2.

From what you copy-pasted, I guess that you ran a MCMC chain with the Planck low-ell ("lowTEB"), high-ell ("plik"), and lensing ("lensing"). The "prior" chi2 is, you guessed it, the contribution coming from the priors you have chosen. The "CMB" chi2 is just the sum of all CMB-related chi2, i.e. "lowTEB+plik+lensing".

You should most likely use the total chi2/likelihood when comparing models...

Cheers,
Stephane