BEAST - ESS of combined logs vs ESS of individual run logs

Sometimes, when running BEAST, it is seen that ESS values of the individual runs is not > 200 but the combined ESS of all the independent runs is > 200 and traces of three independent runs converge in the combined trace. Is it acceptable then to use combined estimate of parameters such as mutation rate?

Here is an answer from BEAST author:

https://groups.google.com/g/beast-users/c/b11vV19nqkI

User:

Is it acceptable to use an estimate of say, TMRCA, from the combined analysis of three identical runs (from different random seeds) if the ESS is >200 for all parameters in the combined file (but not in each individual file) in Tracer?

Or do I need to do that three times (for a total of 9 runs) and use log combiner to combine the nine files?

alexei....@gmail.com

3 runs combined to give an ESS of >200 is probably safe but you need to be a little bit careful. 10 runs to get an ESS of >200 is probably not safe because if individual runs are given ESS estimates of about 20 then there is a question of whether those ESS estimates are valid at all. You should use Tracer to visually inspect that your 3 runs are giving essentially the same answers for all the parameters and the likelihood and prior. The traces should substantially overlap and given basically the same mean and variance. If your three runs give traces that don't overlap then you can't combine them no matter what the combined ESS says! 

Cheers

Alexei