# Weights

In machine learning it is possible to assign each observation an independent significance, or weight, either in training or in performance evaluation, or both.

There are two kinds of weights in use in MLJ:

• per observation weights (also just called weights) refer to weight vectors of the same length as the number of observations

• class weights refer to dictionaries keyed on the target classes (levels) for use in classification problems

## Specifying weights in training

To specify weights in training you bind the weights to the model along with the data when constructing a machine. For supervised models the weights are specified last:

KNNRegressor = @load KNNRegressor
model = KNNRegressor()
X, y = make_regression(10, 3)
w = rand(length(y))

mach = machine(model, X, y, w) |> fit!

Note that model supports per observation weights if supports_weights(model) is true. To list all such models, do

models() do m
m.supports_weights
end

The model model supports class weights if supports_class_weights(model) is true.

## Specifying weights in performance evaluation

When calling an MLJ measure (metric) that supports weights, provide the weights as the last argument, as in

_, y = @load_iris
ŷ = shuffle(y)
w = Dict("versicolor" => 1, "setosa" => 2, "virginica"=> 3)
macro_f1score(ŷ, y, w)

You can use supports_weights and supports_class_weights on measures to check weight support. For example, to list all measures supporting per observation weights, do

measures() do m
m.supports_weights
end 

To pass weights to all the measures listed in an evaluate!/evaluate call, use the keyword specifiers weights=... or class_weights=.... For details, see evaluate!.