Estimate parameter values using a sampling from a datasetΒΆ

Section author: Gavin Huttley

This script uses the sample method of the alignment class to provide an estimate for a two stage optimisation. This allows rapid optimisation of long alignments and complex models with a good chance of arriving at the global maximum for the model and data. Local optimisation of the full alignment may end up in local maximum and for this reason results from this strategy my be inaccurate.

From cogent import all the components we need.

>>> from cogent import LoadSeqs, LoadTree
>>> from cogent.evolve import  substitution_model

Load your alignment, note that if your file ends with a suffix that is the same as it’s format (assuming it’s a supported format) then you can just give the filename. Otherwise you can specify the format using the format argument.

>>> aln = LoadSeqs(filename = "data/long_testseqs.fasta")

Get your tree

>>> t = LoadTree(filename = "data/test.tree")

Get the substitution model (defaults to Felsensteins 1981 model)

>>> sm = substitution_model.Nucleotide()

Make a likelihood function from a sample of the alignment the sample method selects the chosen number of bases at random.

>>> lf = sm.makeLikelihoodFunction(t)
>>> lf.setMotifProbsFromData(aln)
>>> lf.setAlignment(aln.sample(20))

Optimise with the local optimiser

>>> lf.optimise(local=True)

Next use the whole alignment

>>> lf.setAlignment(aln)

and the faster Powell optimiser that will only find the best result near the provided starting point

>>> lf.optimise(local=True)
>>> print lf
Likelihood Function Table
     edge    parent    length
    Human    edge.0    0.0309
HowlerMon    edge.0    0.0412
   edge.0    edge.1    0.0359
    Mouse    edge.1    0.2666
   edge.1      root    0.0226
NineBande      root    0.0895
 DogFaced      root    0.1095
motif    mprobs
    T    0.2317
    C    0.1878
    A    0.3681
    G    0.2125