kfoldLoss
Classification loss for crossvalidated classification model
Description
returns the classification loss obtained by the crossvalidated classification model
L
= kfoldLoss(CVMdl
)CVMdl
. For every fold, kfoldLoss
computes the
classification loss for validationfold observations using a classifier trained on
trainingfold observations. CVMdl.X
and CVMdl.Y
contain both sets of observations.
returns the classification loss with additional options specified by one or more namevalue
arguments. For example, you can specify a custom loss function.L
= kfoldLoss(CVMdl
,Name,Value
)
Examples
Estimate CrossValidated Classification Error
Load the ionosphere
data set.
load ionosphere
Grow a classification tree.
tree = fitctree(X,Y);
Crossvalidate the classification tree using 10fold crossvalidation.
cvtree = crossval(tree);
Estimate the crossvalidated classification error.
L = kfoldLoss(cvtree)
L = 0.1083
Estimate CrossValidated Classification Error
Load the ionosphere
data set.
load ionosphere
Train a classification ensemble of 100 decision trees using AdaBoostM1. Specify tree stumps as the weak learners.
t = templateTree('MaxNumSplits',1); ens = fitcensemble(X,Y,'Method','AdaBoostM1','Learners',t);
Crossvalidate the ensemble using 10fold crossvalidation.
cvens = crossval(ens);
Estimate the crossvalidated classification error.
L = kfoldLoss(cvens)
L = 0.0655
Find Optimal Number of Trees for GAM Using kfoldLoss
Train a crossvalidated generalized additive model (GAM) with 10 folds. Then, use kfoldLoss
to compute cumulative crossvalidation classification errors (misclassification rate in decimal). Use the errors to determine the optimal number of trees per predictor (linear term for predictor) and the optimal number of trees per interaction term.
Alternatively, you can find optimal values of fitcgam
namevalue arguments by using the OptimizeHyperparameters namevalue argument. For an example, see Optimize GAM Using OptimizeHyperparameters.
Load the ionosphere
data set. This data set has 34 predictors and 351 binary responses for radar returns, either bad ('b'
) or good ('g'
).
load ionosphere
Create a crossvalidated GAM by using the default crossvalidation option. Specify the 'CrossVal'
namevalue argument as 'on'
. Specify to include all available interaction terms whose pvalues are not greater than 0.05.
rng('default') % For reproducibility CVMdl = fitcgam(X,Y,'CrossVal','on','Interactions','all','MaxPValue',0.05);
If you specify 'Mode'
as 'cumulative'
for kfoldLoss
, then the function returns cumulative errors, which are the average errors across all folds obtained using the same number of trees for each fold. Display the number of trees for each fold.
CVMdl.NumTrainedPerFold
ans = struct with fields:
PredictorTrees: [65 64 59 61 60 66 65 62 64 61]
InteractionTrees: [1 2 2 2 2 1 2 2 2 2]
kfoldLoss
can compute cumulative errors using up to 59 predictor trees and one interaction tree.
Plot the cumulative, 10fold crossvalidated, classification error (misclassification rate in decimal). Specify 'IncludeInteractions'
as false
to exclude interaction terms from the computation.
L_noInteractions = kfoldLoss(CVMdl,'Mode','cumulative','IncludeInteractions',false); figure plot(0:min(CVMdl.NumTrainedPerFold.PredictorTrees),L_noInteractions)
The first element of L_noInteractions
is the average error over all folds obtained using only the intercept (constant) term. The (J+1
)th element of L_noInteractions
is the average error obtained using the intercept term and the first J
predictor trees per linear term. Plotting the cumulative loss allows you to monitor how the error changes as the number of predictor trees in GAM increases.
Find the minimum error and the number of predictor trees used to achieve the minimum error.
[M,I] = min(L_noInteractions)
M = 0.0655
I = 23
The GAM achieves the minimum error when it includes 22 predictor trees.
Compute the cumulative classification error using both linear terms and interaction terms.
L = kfoldLoss(CVMdl,'Mode','cumulative')
L = 2×1
0.0712
0.0712
The first element of L
is the average error over all folds obtained using the intercept (constant) term and all predictor trees per linear term. The second element of L
is the average error obtained using the intercept term, all predictor trees per linear term, and one interaction tree per interaction term. The error does not decrease when interaction terms are added.
If you are satisfied with the error when the number of predictor trees is 22, you can create a predictive model by training the univariate GAM again and specifying 'NumTreesPerPredictor',22
without crossvalidation.
Input Arguments
CVMdl
— Crossvalidated partitioned classifier
ClassificationPartitionedModel
object  ClassificationPartitionedEnsemble
object  ClassificationPartitionedGAM
object
Crossvalidated partitioned classifier, specified as a ClassificationPartitionedModel
, ClassificationPartitionedEnsemble
, or ClassificationPartitionedGAM
object. You can create the object in two ways:
Pass a trained classification model listed in the following table to its
crossval
object function.Train a classification model using a function listed in the following table and specify one of the crossvalidation namevalue arguments for the function.
NameValue Arguments
Specify optional pairs of arguments as
Name1=Value1,...,NameN=ValueN
, where Name
is
the argument name and Value
is the corresponding value.
Namevalue arguments must appear after other arguments, but the order of the
pairs does not matter.
Before R2021a, use commas to separate each name and value, and enclose
Name
in quotes.
Example: kfoldLoss(CVMdl,'Folds',[1 2 3 5])
specifies to use the
first, second, third, and fifth folds to compute the classification loss, but to exclude the
fourth fold.
Folds
— Fold indices to use
1:CVMdl.KFold
(default)  positive integer vector
Fold indices to use, specified as a positive integer vector. The elements of Folds
must be within the range from 1
to CVMdl.KFold
.
The software uses only the folds specified in Folds
.
Example: 'Folds',[1 4 10]
Data Types: single
 double
IncludeInteractions
— Flag to include interaction terms
true
 false
Flag to include interaction terms of the model, specified as true
or
false
. This argument is valid only for a generalized
additive model (GAM). That is, you can specify this argument only when
CVMdl
is ClassificationPartitionedGAM
.
The default value is true
if the models in
CVMdl
(CVMdl.Trained
) contain
interaction terms. The value must be false
if the models do not
contain interaction terms.
Data Types: logical
LossFun
— Loss function
'binodeviance'
 'classifcost'
 'classiferror'
 'crossentropy'
 'exponential'
 'hinge'
 'logit'
 'mincost'
 'quadratic'
 function handle
Loss function, specified as a builtin loss function name or a function handle.
The default loss function depends on the model type of CVMdl
.
The default value is
'classiferror'
if the model type is an ensemble or support vector machine classifier.The default value is
'mincost'
if the model type is a discriminant analysis, knearest neighbor, naive Bayes, neural network, or tree classifier.If the model type is a generalized additive model classifier, the default value is
'mincost'
if theScoreTransform
property of the input model object (
) isCVMdl
.ScoreTransform'logit'
; otherwise, the default value is'classiferror'
.
'classiferror'
and 'mincost'
are
equivalent when you use the default cost matrix. See Algorithms for more information.
This table lists the available loss functions. Specify one using its corresponding character vector or string scalar.
Value Description 'binodeviance'
Binomial deviance 'classifcost'
Observed misclassification cost 'classiferror'
Misclassified rate in decimal 'crossentropy'
Crossentropy loss (for neural networks only) 'exponential'
Exponential loss 'hinge'
Hinge loss 'logit'
Logistic loss 'mincost'
Minimal expected misclassification cost (for classification scores that are posterior probabilities) 'quadratic'
Quadratic loss 'mincost'
is appropriate for classification scores that are posterior probabilities. Thepredict
andkfoldPredict
functions of discriminant analysis, generalized additive model, knearest neighbor, naive Bayes, neural network, and tree classifiers return such scores by default.For ensemble models that use
'Bag'
or'Subspace'
methods, classification scores are posterior probabilities by default. For ensemble models that use'AdaBoostM1'
,'AdaBoostM2'
,GentleBoost
, or'LogitBoost'
methods, you can use posterior probabilities as classification scores by specifying the doublelogit score transform. For example, enter:For all other ensemble methods, the software does not support posterior probabilities as classification scores.CVMdl.ScoreTransform = 'doublelogit';
For SVM models, you can specify to use posterior probabilities as classification scores by setting
'FitPosterior',true
when you crossvalidate the model usingfitcsvm
.
Specify your own function using function handle notation.
Suppose that
n
is the number of observations in the training data (CVMdl.NumObservations
) andK
is the number of classes (numel(CVMdl.ClassNames)
). Your function must have the signaturelossvalue =
, where:lossfun
(C,S,W,Cost)The output argument
lossvalue
is a scalar.You specify the function name (
lossfun
).C
is ann
byK
logical matrix with rows indicating the class to which the corresponding observation belongs. The column order corresponds to the class order inCVMdl.ClassNames
.Construct
C
by settingC(p,q) = 1
if observationp
is in classq
, for each row. Set all other elements of rowp
to0
.S
is ann
byK
numeric matrix of classification scores. The column order corresponds to the class order inCVMdl.ClassNames
. The inputS
resembles the output argumentscore
ofkfoldPredict
.W
is ann
by1 numeric vector of observation weights. If you passW
, the software normalizes its elements to sum to1
.Cost
is aK
byK
numeric matrix of misclassification costs. For example,Cost = ones(K) – eye(K)
specifies a cost of0
for correct classification, and1
for misclassification.
Specify your function using
'LossFun',@
.lossfun
For more details on loss functions, see Classification Loss.
Example: 'LossFun','hinge'
Data Types: char
 string
 function_handle
Mode
— Aggregation level for output
'average'
(default)  'individual'
 'cumulative'
Aggregation level for the output, specified as 'average'
, 'individual'
, or 'cumulative'
.
Value  Description 

'average'  The output is a scalar average over all folds. 
'individual'  The output is a vector of length k containing one value per fold, where k is the number of folds. 
'cumulative'  Note If you want to specify this value,

Example: 'Mode','individual'
Output Arguments
L
— Classification loss
numeric scalar  numeric column vector
Classification loss, returned as a numeric scalar or numeric column vector.
If
Mode
is'average'
, thenL
is the average classification loss over all folds.If
Mode
is'individual'
, thenL
is a kby1 numeric column vector containing the classification loss for each fold, where k is the number of folds.If
Mode
is'cumulative'
andCVMdl
isClassificationPartitionedEnsemble
, thenL
is amin(CVMdl.NumTrainedPerFold)
by1 numeric column vector. Each elementj
is the average classification loss over all folds that the function obtains by using ensembles trained with weak learners1:j
.If
Mode
is'cumulative'
andCVMdl
isClassificationPartitionedGAM
, then the output value depends on theIncludeInteractions
value.If
IncludeInteractions
isfalse
, thenL
is a(1 + min(NumTrainedPerFold.PredictorTrees))
by1 numeric column vector. The first element ofL
is the average classification loss over all folds that is obtained using only the intercept (constant) term. The(j + 1)
th element ofL
is the average loss obtained using the intercept term and the firstj
predictor trees per linear term.If
IncludeInteractions
istrue
, thenL
is a(1 + min(NumTrainedPerFold.InteractionTrees))
by1 numeric column vector. The first element ofL
is the average classification loss over all folds that is obtained using the intercept (constant) term and all predictor trees per linear term. The(j + 1)
th element ofL
is the average loss obtained using the intercept term, all predictor trees per linear term, and the firstj
interaction trees per interaction term.
More About
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 (or the first or second class in the
ClassNames
property), respectively.f(X_{j}) is the positiveclass classification score for observation (row) j of the predictor data X.
m_{j} = y_{j}f(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 y_{2}^{*} = [
0 0 1 0
]′. The order of the classes corresponds to the order in theClassNames
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 stored in the
Prior
property. 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
namevalue argument.
Loss Function  Value of LossFun  Equation 

Binomial deviance  'binodeviance'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}\mathrm{log}\left\{1+\mathrm{exp}\left[2{m}_{j}\right]\right\}}.$$ 
Observed misclassification cost  'classifcost'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}}{c}_{{y}_{j}{\widehat{y}}_{j}},$$ where $${\widehat{y}}_{j}$$ is the class label corresponding to the class with the maximal score, and $${c}_{{y}_{j}{\widehat{y}}_{j}}$$ is the userspecified cost of classifying an observation into class $${\widehat{y}}_{j}$$ when its true class is y_{j}. 
Misclassified rate in decimal  'classiferror'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}}I\left\{{\widehat{y}}_{j}\ne {y}_{j}\right\},$$ where I{·} is the indicator function. 
Crossentropy loss  'crossentropy' 
The weighted crossentropy loss is $$L={\displaystyle \sum _{j=1}^{n}\frac{{\tilde{w}}_{j}\mathrm{log}({m}_{j})}{Kn}},$$ where the weights $${\tilde{w}}_{j}$$ are normalized to sum to n instead of 1. 
Exponential loss  'exponential'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}\mathrm{exp}\left({m}_{j}\right)}.$$ 
Hinge loss  'hinge'  $$L={\displaystyle \sum}_{j=1}^{n}{w}_{j}\mathrm{max}\left\{0,1{m}_{j}\right\}.$$ 
Logit loss  'logit'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}\mathrm{log}\left(1+\mathrm{exp}\left({m}_{j}\right)\right)}.$$ 
Minimal expected misclassification cost  'mincost' 
The software computes the weighted minimal expected classification cost using this procedure for observations j = 1,...,n.
The weighted average of the minimal expected misclassification cost loss is $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}{c}_{j}}.$$ 
Quadratic loss  'quadratic'  $$L={\displaystyle \sum _{j=1}^{n}{w}_{j}{\left(1{m}_{j}\right)}^{2}}.$$ 
If you use the default cost matrix (whose element value is 0 for correct classification
and 1 for incorrect classification), then the loss values for
'classifcost'
, 'classiferror'
, and
'mincost'
are identical. For a model with a nondefault cost matrix,
the 'classifcost'
loss is equivalent to the 'mincost'
loss most of the time. These losses can be different if prediction into the class with
maximal posterior probability is different from prediction into the class with minimal
expected cost. Note that 'mincost'
is appropriate only if classification
scores are posterior probabilities.
This figure compares the loss functions (except 'classifcost'
,
'crossentropy'
, and 'mincost'
) over the score
m for one observation. Some functions are normalized to pass through
the point (0,1).
Algorithms
kfoldLoss
computes the classification loss as described in the
corresponding loss
object function. For a modelspecific description, see
the appropriate loss
function reference page in the following
table.
Model Type  loss Function 

Discriminant analysis classifier  loss 
Ensemble classifier  loss 
Generalized additive model classifier  loss 
knearest neighbor classifier  loss 
Naive Bayes classifier  loss 
Neural network classifier  loss 
Support vector machine classifier  loss 
Binary decision tree for multiclass classification  loss 
Extended Capabilities
GPU Arrays
Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.
Usage notes and limitations:
This function fully supports GPU arrays for the following crossvalidated model objects:
Ensemble classifier trained with
fitcensemble
knearest neighbor classifier trained with
fitcknn
Support vector machine classifier trained with
fitcsvm
Binary decision tree for multiclass classification trained with
fitctree
For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).
Version History
Introduced in R2011aR2022a: kfoldLoss
returns a different value for crossvalidated SVM and ensemble classifiers with a nondefault cost matrix
If you specify a nondefault cost matrix when you crossvalidate the input model object for an SVM or ensemble classification model, the kfoldLoss
function returns a different value compared to previous releases.
The kfoldLoss
function uses the
observation weights stored in the W
property. Also, the function uses the
cost matrix stored in the Cost
property if you specify the
LossFun
namevalue argument as "classifcost"
or
"mincost"
. The way the function uses the W
and
Cost
property values has not changed. However, the property values stored in the input model object have changed for
crossvalidated SVM and ensemble model objects with a nondefault cost matrix, so the
function can return a different value.
For details about the property value change, see Cost property stores the userspecified cost matrix (crossvalidated SVM classifier) or Cost property stores the userspecified cost matrix (crossvalidated ensemble classifier).
If you want the software to handle the cost matrix, prior
probabilities, and observation weights as in previous releases, adjust the prior probabilities
and observation weights for the nondefault cost matrix, as described in Adjust Prior Probabilities and Observation Weights for Misclassification Cost Matrix. Then, when you train a
classification model, specify the adjusted prior probabilities and observation weights by using
the Prior
and Weights
namevalue arguments, respectively,
and use the default cost matrix.
R2022a: Default LossFun
value has changed for crossvalidated GAM and neural network classifiers
Starting in R2022a, the default value of the LossFun
namevalue
argument has changed for both a generalized additive model (GAM) and a neural network model,
so that the kfoldLoss
function uses the "mincost"
option (minimal expected misclassification cost) as the default when a crossvalidated
classification object uses posterior probabilities for classification scores.
If the model type of the input model object
CVMdl
is a GAM classifier, the default value is"mincost"
if theScoreTransform
property ofCVMdl
(
) isCVMdl
.ScoreTransform'logit'
; otherwise, the default value is"classiferror"
.If the model type of
CVMdl
is a neural network model classifier, the default value is"mincost"
.
In previous releases, the default value was
'classiferror'
.
You do not need to make any changes to your code if you use the default cost matrix (whose element value is 0 for correct classification and 1 for incorrect classification). The "mincost"
option is equivalent to the "classiferror"
option for the default cost matrix.
See Also
ClassificationPartitionedModel
 kfoldPredict
 kfoldEdge
 kfoldMargin
 kfoldfun
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