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Error "Error: no valid set of coefficients has been found: please supply starting values" for model that runs OK in nnet::multinom #28
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The warning about the missing starting values occurs after the 8-th iteration - where it should not occur. This is a bug that I am able to fix. Another bug is that there actually is no way to provide starting values. I am going to fix this in the next days. The real issue here is that the algorithm diverges in the present situation. I am able to tweak the algorithm so that it converges with the help of some stepsize-halving. However, this is a "false convergence" in so far as the algorithm does not stop at a true optimum of the objective function. This is all because the objective function does not seem to have an optimum at all, because we have an instance of separation at hand and the algorithm should not converge because regular ML estimates do not exist. If I inspect the results of
With 7356b69 I get with your model and the data after 27 iterations:
and
The deviance that |
Many thanks for the info! So it's a case with complete separation. Do you think anything could be added to deal with such cases a bit better? E.g. allowing one to add a small ridge or adaptive ridge penalty (equivalent to adding a Gaussian prior) to make such models fit? (I recall that with binomial GLMs one can add a ridge penalty by row augmenting the model matrix with a diagonal matrix with sqrt(lambdas) along the diagonal and augmenting the observations with some zeros - I presume the same is possible in the context of a multinomial GLM? I see a ridge pentalty being used here e.g., https://ieeexplore.ieee.org/abstract/document/1424458, aside from other penalties that promote sparsity (which is also cool). Haven't seen any ports of that algorithm actually in R - though there are some in Matlab, C & .NET |
Recently bumped into a model for which no valid set of coefficients could be found, even though the model runs OK in
nnet::multinom
:Is there any way to pass starting coefficients actually?
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