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EvoTreeClassifier

EvoTreeClassifier(;kwargs...)

A model type for constructing a EvoTreeClassifier, based on EvoTrees.jl, and implementing both an internal API and the MLJ model interface. EvoTreeClassifier is used to perform multi-class classification, using cross-entropy loss.

Hyper-parameters

  • early_stopping_rounds::Integer: number of consecutive rounds without metric improvement after which fitting in stopped.

  • nrounds=100: Number of rounds. It corresponds to the number of trees that will be sequentially stacked. Must be >= 1.

  • eta=0.1: Learning rate. Each tree raw predictions are scaled by eta prior to be added to the stack of predictions. Must be > 0. A lower eta results in slower learning, requiring a higher nrounds but typically improves model performance.

  • L2::T=0.0: L2 regularization factor on aggregate gain. Must be >= 0. Higher L2 can result in a more robust model.

  • lambda::T=0.0: L2 regularization factor on individual gain. Must be >= 0. Higher lambda can result in a more robust model.

  • gamma::T=0.0: Minimum gain improvement needed to perform a node split. Higher gamma can result in a more robust model. Must be >= 0.

  • max_depth=6: Maximum depth of a tree. Must be >= 1. A tree of depth 1 is made of a single prediction leaf. A complete tree of depth N contains 2^(N - 1) terminal leaves and 2^(N - 1) - 1 split nodes. Compute cost is proportional to 2^max_depth. Typical optimal values are in the 3 to 9 range.

  • min_weight=1.0: Minimum weight needed in a node to perform a split. Matches the number of observations by default or the sum of weights as provided by the weights vector. Must be > 0.

  • rowsample=1.0: Proportion of rows that are sampled at each iteration to build the tree. Should be in ]0, 1].

  • colsample=1.0: Proportion of columns / features that are sampled at each iteration to build the tree. Should be in ]0, 1].

  • nbins=64: Number of bins into which each feature is quantized. Buckets are defined based on quantiles, hence resulting in equal weight bins. Should be between 2 and 255.

  • tree_type=:binary Tree structure to be used. One of:

    • :binary: Each node of a tree is grown independently. Tree are built depthwise until max depth is reach or if min weight or gain (see gamma) stops further node splits.
    • :oblivious: A common splitting condition is imposed to all nodes of a given depth.

  • rng=123: Either an integer used as a seed to the random number generator or an actual random number generator (::Random.AbstractRNG).

  • device=:cpu: Hardware device to use for computations. Can be either :cpu or :gpu.

Internal API

Do config = EvoTreeClassifier() to construct an instance with default hyper-parameters. Provide keyword arguments to override hyper-parameter defaults, as in EvoTreeClassifier(max_depth=...).

Training model

A model is built using fit_evotree:

model = fit_evotree(config; x_train, y_train, kwargs...)

Inference

Predictions are obtained using predict which returns a Matrix of size [nobs, K] where K is the number of classes:

EvoTrees.predict(model, X)

Alternatively, models act as a functor, returning predictions when called as a function with features as argument:

model(X)

MLJ

From MLJ, the type can be imported using:

EvoTreeClassifier = @load EvoTreeClassifier pkg=EvoTrees

Do model = EvoTreeClassifier() to construct an instance with default hyper-parameters. Provide keyword arguments to override hyper-parameter defaults, as in EvoTreeClassifier(loss=...).

Training data

In MLJ or MLJBase, bind an instance model to data with

mach = machine(model, X, y)

where

  • X: any table of input features (eg, a DataFrame) whose columns each have one of the following element scitypes: Continuous, Count, or <:OrderedFactor; check column scitypes with schema(X)
  • y: is the target, which can be any AbstractVector whose element scitype is <:Multiclas or <:OrderedFactor; check the scitype with scitype(y)

Train the machine using fit!(mach, rows=...).

Operations

  • predict(mach, Xnew): return predictions of the target given features Xnew having the same scitype as X above. Predictions are probabilistic.
  • predict_mode(mach, Xnew): returns the mode of each of the prediction above.

Fitted parameters

The fields of fitted_params(mach) are:

  • :fitresult: The GBTree object returned by EvoTrees.jl fitting algorithm.

Report

The fields of report(mach) are:

  • :features: The names of the features encountered in training.

Examples

## Internal API
using EvoTrees
config = EvoTreeClassifier(max_depth=5, nbins=32, nrounds=100)
nobs, nfeats = 1_000, 5
x_train, y_train = randn(nobs, nfeats), rand(1:3, nobs)
model = fit_evotree(config; x_train, y_train)
preds = EvoTrees.predict(model, x_train)
## MLJ Interface
using MLJ
EvoTreeClassifier = @load EvoTreeClassifier pkg=EvoTrees
model = EvoTreeClassifier(max_depth=5, nbins=32, nrounds=100)
X, y = @load_iris
mach = machine(model, X, y) |> fit!
preds = predict(mach, X)
preds = predict_mode(mach, X)

See also EvoTrees.jl.