# Multinomial logistic regression

Has anyone done multinomial logistic regression in Mathematica?

The binomial case is essentially done on the LogitModelFit documentation page and works fine.

I am following this formulation to obtain the outcome probabilities for k classes with k-1 calls to LogitModelFit.

I am getting normalized probabilities out, but they depend on how the classes are encoded for each independent regression. I can post what I have on request but it's a lot of code for something that isn't working.

Either multiple-binary or true multinomial would be fine for my purposes, but bonus for both implementations.

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Multiple-Binary Approach

Lets bring out the famous iris data set to set this up and split it into 80% training and 20% testing data to be used in fitting the model.

iris = ExampleData[{"Statistics", "FisherIris"}];
n = Length[iris]

rs = RandomSample[Range[n]];
cut = Ceiling[.8 n];
train = iris[[rs[[1 ;; cut]]]];
test = iris[[rs[[cut + 1 ;;]]]];

speciesList = Union[iris[[All,-1]]];


First we need to run a separate regression for each species of iris. I'm creating an encoder for the response for this purpose which gives 1 if the response matches a particular species and 0 otherwise.

encodeResponse[data_, species_] := Block[{resp},
resp = data[[All, -1]];
Join[data[[All, 1 ;; -2]]\[Transpose], {Boole[# === species] & /@
resp}]\[Transpose]
]


Now we fit a model for each of the three species using this encoded training data.

logmods =
Table[LogitModelFit[encodeResponse[train, i], Array[x, 4],
Array[x, 4]], {i, speciesList}];


Each model can be used to estimate a probability that a particular set of features yields its class. For classification, we simply let them all "vote" and pick the category with highest probability.

mlogitClassify[mods_, speciesList_][x_] :=
Block[{p},
p = #[Sequence @@ x] & /@ mods;
speciesList[[Ordering[p, -1][[1]]]]
]


So how well did this perform? In this case we get 14 out of 15 correct on the testing data (pretty good!).

class = mlogitClassify[logmods, speciesList][#] & /@ test[[All, 1 ;; -2]];

(* 14/15 *)


It is interesting to see what this classifier actually does visually. To do so, I'll reduce the number of variables to 2.

logmods2 =
Table[LogitModelFit[encodeResponse[train[[All, {3, 4, 5}]], i],
Array[x, 2], Array[x, 2]], {i, speciesList}];

Show[Table[
RegionPlot[
mlogitClassify[logmods2, speciesList][{x1, x2}] === i, {x1, .5,
7}, {x2, 0, 2.5},
PlotStyle ->
Directive[Opacity[.25],
Switch[i, "setosa", Red, "virginica", Green, "versicolor",
Blue]]], {i, {"setosa", "virginica", "versicolor"}}],
ListPlot[Table[
Tooltip[Pick[iris[[All, {3, 4}]], iris[[All, -1]], i],
i], {i, {"setosa", "virginica", "versicolor"}}],
PlotStyle -> {Red, Darker[Green], Blue}]]


The decision boundaries are more interesting if we allow more flexibility in the basis functions. Here I allow all of the possible quadratic terms.

logmods2 =
Table[LogitModelFit[
encodeResponse[train[[All, {3, 4, 5}]], i], {1, x[1], x[2],
x[1]^2, x[2]^2, x[1] x[2]}, Array[x, 2]], {i, speciesList}];

Show[Table[
RegionPlot[
mlogitClassify[logmods2, speciesList][{x1, x2}] === i, {x1, .5,
7}, {x2, 0, 2.5},
PlotStyle ->
Directive[Opacity[.25],
Switch[i, "setosa", Red, "virginica", Green, "versicolor",
Blue]], PlotPoints -> 100], {i, {"setosa", "virginica",
"versicolor"}}],
ListPlot[Table[
Tooltip[Pick[iris[[All, {3, 4}]], iris[[All, -1]], i],
i], {i, {"setosa", "virginica", "versicolor"}}],
PlotStyle -> {Red, Darker[Green], Blue}]]


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Good stuff. How/when/why would you choose non-standard basis functions? When you had some physical reason to believe the response had e.g. quadratic dependence on one of the variables? – Ibn Opcit Mar 29 '13 at 13:48
@IbnOpcit that is certainly one reason. Another is that if classification is all you care about (i.e. not the parameter estimates) it is not atypical to do this to give the learning method more freedom to fit the data. The big problem is that it can easily lead to over fitting so some sort of regularization method like lasso or ridge needs to be used and unfortunately M doesn't allow this automatically in LogitModelFit. – Andy Ross Mar 29 '13 at 14:59
@IbnOpcit that's not to say that regularized logistic regression isn't straightforward in M, in fact I do this often. The hard part is writing your own iteratively reweighted least squares code in the case of ridge regression and dealing with (in my experience) poor scalability with FindMinimum or some such for lasso. – Andy Ross Mar 29 '13 at 15:43