## Bactrian Scale Operator

Scale operator that finds scale factor according to a Bactrian distribution (Yang & Rodriguez, 2013), which is a mixture of two Gaussians: p(x) = 1/2*N(x;-m,1-m^2) + 1/2*N(x;+m,1-m^2) and more efficient than non-Bactrian scale operator.

`kernelDistribution`

(KernelDistribution)

Provides sample distribution for proposals (optional, default: bactrian) See also kernel distributions.

`tree`

(Tree)

Beast tree for wich divergence times are scaled.

`scaleFactor`

(Double)

Magnitude factor used for scaling.
Larger values mean bolder proposals, but higher chance of rejection.
When `optimise`

is set to true, this value is automatically tuned during MCMC.
A starting value of 0.1 seems to be OK in that case.

`rootOnly`

(Boolean)

Scale root of a tree only, ignored if tree is not specified.
For scaling the whole tree, the epoch flex and tree stretch operators are more efficient, so `rootOnly=true`

is recommended.

`optimise`

(Boolean)

Flag to indicate that the scale factor is automatically changed in order to achieve a good acceptance rate (default true, recommended).

`upper`

(Double)

Upper Limit of scale factor (optional, default: 0.99999999).

`lower`

(Double)

Lower limit of scale factor (optional, default: 1.0E-8).

`weight`

(Double)

Weight with which this operator is selected (required). See also operator weight tuning

## References

Yang Z, Rodríguez CE. Searching for efficient Markov chain Monte Carlo proposal kernels. Proceedings of the National Academy of Sciences. 2013 Nov 26;110(48):19307-12. doi: 10.1073/pnas.1311790110.