kfoldLoss

Classification loss for observations not used for training

Syntax

L = kfoldLoss(obj) L = kfoldLoss(obj,Name,Value)

Description

L = kfoldLoss(obj) returns loss obtained by cross-validated classification model obj. For every fold, this method computes classification loss for in-fold observations using a model trained on out-of-fold observations.

L = kfoldLoss(obj,Name,Value) calculates loss with additional options specified by one or more Name,Value pair arguments. You can specify several name-value pair arguments in any order as Name1,Value1,…,NameN,ValueN.

Input Arguments

obj

Object of class ClassificationPartitionedModel.

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside quotes. You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

'folds'

Indices of folds ranging from 1 to obj.KFold. Use only these folds for predictions.

Default: 1:obj.KFold

'lossfun'

Loss function, specified as the comma-separated pair consisting of 'LossFun' and a built-in, loss-function name or function handle.

The following table lists the available loss functions. Specify one using its corresponding character vector or string scalar.

Value	Description
`'binodeviance'`	Binomial deviance
`'classiferror'`	Classification error
`'exponential'`	Exponential
`'hinge'`	Hinge
`'logit'`	Logistic
`'mincost'`	Minimal expected misclassification cost (for classification scores that are posterior probabilities)
`'quadratic'`	Quadratic

'mincost' is appropriate for classification scores that are posterior probabilities. All models use posterior probabilities as classification scores by default except SVM models. You can specify to use posterior probabilities as classification scores for SVM models by setting 'FitPosterior',true when you cross-validate the model using fitcsvm.

Specify your own function using function handle notation.
Suppose that n be the number of observations in X and K be the number of distinct classes (numel(obj.ClassNames), obj is the input model). Your function must have this signature
```
lossvalue = lossfun(C,S,W,Cost)
```
where:
- The output argument lossvalue is a scalar.
- You choose the function name (lossfun).
- C is an n-by-K logical matrix with rows indicating which class the corresponding observation belongs. The column order corresponds to the class order in obj.ClassNames.
  Construct C by setting C(p,q) = 1 if observation p is in class q, for each row. Set all other elements of row p to 0.
- S is an n-by-K numeric matrix of classification scores. The column order corresponds to the class order in obj.ClassNames. S is a matrix of classification scores, similar to the output of predict.
- W is an n-by-1 numeric vector of observation weights. If you pass W, the software normalizes them to sum to 1.
- Cost is a K-by-K numeric matrix of misclassification costs. For example, Cost = ones(K) - eye(K) specifies a cost of 0 for correct classification, and 1 for misclassification.
Specify your function using 'LossFun',@lossfun.

For more details on loss functions, see Classification Loss.

Default: 'classiferror'

'mode'

A character vector or string scalar for determining the output of kfoldLoss:

'average' — L is a scalar, the loss averaged over all folds.
'individual' — L is a vector of length obj.KFold, where each entry is the loss for a fold.

Default: 'average'

Output Arguments

`L`	Loss, by default the fraction of misclassified data. `L` can be a vector, and can mean different things, depending on the name-value pair settings.

Examples

expand all

Estimate Cross-Validated Classification Error

Open Live Script

Load the ionosphere data set.

load ionosphere

Grow a classification tree.

tree = fitctree(X,Y);

Cross-validate the classification tree using 10-fold cross-validation.

cvtree = crossval(tree);

Estimate the cross-validated classification error.

L = kfoldLoss(cvtree)

L = 0.1083

More About

expand all

Classification Loss

Classification loss functions measure the predictive inaccuracy of classification models. When you compare the same type of loss among many models, a lower loss indicates a better predictive model.

Consider the following scenario.

L is the weighted average classification loss.
n is the sample size.
For binary classification:
- y_j is the observed class label. The software codes it as –1 or 1, indicating the negative or positive class, respectively.
- f(X_j) is the raw classification score for observation (row) j of the predictor data X.
- m_j = y_jf(X_j) is the classification score for classifying observation j into the class corresponding to y_j. Positive values of m_j indicate correct classification and do not contribute much to the average loss. Negative values of m_j indicate incorrect classification and contribute significantly to the average loss.
For algorithms that support multiclass classification (that is, K ≥ 3):
- y_j^* is a vector of K – 1 zeros, with 1 in the position corresponding to the true, observed class y_j. For example, if the true class of the second observation is the third class and K = 4, then y₂^* = [0 0 1 0]′. The order of the classes corresponds to the order in the ClassNames property of the input model.
- f(X_j) is the length K vector of class scores for observation j of the predictor data X. The order of the scores corresponds to the order of the classes in the ClassNames property of the input model.
- m_j = y_j^*′f(X_j). Therefore, m_j is the scalar classification score that the model predicts for the true, observed class.
The weight for observation j is w_j. The software normalizes the observation weights so that they sum to the corresponding prior class probability. The software also normalizes the prior probabilities so they sum to 1. Therefore,

$\sum_{j = 1}^{n} w_{j} = 1.$

Given this scenario, the following table describes the supported loss functions that you can specify by using the 'LossFun' name-value pair argument.

Loss Function	Value of `LossFun`	Equation
Binomial deviance	`'binodeviance'`	$L = \sum_{j = 1}^{n} w_{j} \log {1 + \exp [- 2 m_{j}]} .$
Exponential loss	`'exponential'`	$L = \sum_{j = 1}^{n} w_{j} \exp (- m_{j}) .$
Classification error	`'classiferror'`	$L = \sum_{j = 1}^{n} w_{j} I {{\hat{y}}_{j} \neq y_{j}} .$ The classification error is the weighted fraction of misclassified observations where ${\hat{y}}_{j}$ is the class label corresponding to the class with the maximal posterior probability. I{x} is the indicator function.
Hinge loss	`'hinge'`	$L = \sum_{j = 1}^{n} w_{j} \max {0, 1 - m_{j}} .$
Logit loss	`'logit'`	$L = \sum_{j = 1}^{n} w_{j} \log (1 + \exp (- m_{j})) .$
Minimal cost	`'mincost'`	The software computes the weighted minimal cost using this procedure for observations j = 1,...,n. Estimate the 1-by-K vector of expected classification costs for observation j: $γ_{j} = f {(X_{j})}^{'} C .$ f(X_j) is the column vector of class posterior probabilities for binary and multiclass classification. C is the cost matrix stored by the input model in the `Cost` property. For observation j, predict the class label corresponding to the minimum expected classification cost: ${\hat{y}}_{j} = \min_{j = 1, ..., K} (γ_{j}) .$ Using C, identify the cost incurred (c_j) for making the prediction. The weighted, average, minimum cost loss is $L = \sum_{j = 1}^{n} w_{j} c_{j} .$
Quadratic loss	`'quadratic'`	$L = \sum_{j = 1}^{n} w_{j} {(1 - m_{j})}^{2} .$

This figure compares the loss functions (except 'mincost') for one observation over m. Some functions are normalized to pass through [0,1].

Documentation