I'm confused how to calculate the mean of two (natural) log-likelihood values. The software I'm using (MrBayes) does this for me automatically... sometimes. But in situations when it does not, I need to calculate it myself.

Here is an example:

log-likelihood A: -104520.13

log-likelihood B: -104528.54

software's mean: -104520.83

Here is a different example:

log-likelihood A: -104506.97

log-likelihood B: -104523.28

software's mean: -104507.66

Clearly the mean is not calculated by (A+B)/2.

Thanks much for the advice!

-Steven