Why do I have the same validation accuracy for every epoch?

I developed a CNN for ECG arrhythmia classification and when I train the model I obtain the same validation accuracy for all of the 50 epoch. Can you please tell me what is wrong? I tried to modify the parameters, also the structure, but the validation accuracy is unchanged (80.1%).

Labels=cnnLabels(labels); %Divide and add LABELS in classes from 1 to 5 
[heartbeats, ~, ~] = featureNormalize(heartbeats); % Normalization

%% Spliting the data in training and testing sets
PercentNumFiles =round(0.90*length(heartbeats)); %90% of the files for training and 10% for testing
trainingPercent=round(0.9*PercentNumFiles);%90% of the training set is used for training and the other 10 for training validation

randomNum(:,1)=randperm(length(heartbeats)); %random selection of the files 

Xtrain=heartbeats(randomNum(1:trainingPercent),:);
Xvalidation=heartbeats(randomNum(trainingPercent+1:PercentNumFiles),:);
Xtest=heartbeats(randomNum(PercentNumFiles+1:end),:);

Ytrain=Labels(randomNum(1:trainingPercent));
Yvalidation=Labels(randomNum(trainingPercent+1:PercentNumFiles));
Ytest=Labels(randomNum(PercentNumFiles+1:end));

%% CNN 
clear ECG fs ind before after anntype cleanECG PercentNumFiles randomNum

height = 1;
width = 300;
channels = 1;

Xtrain = reshape(Xtrain,[height, width, channels, length(Xtrain)]);
Xvalidation=reshape(Xvalidation,[height, width, channels, length(Xvalidation)]);
Xtest = reshape(Xtest,[height, width, channels, length(Xtest)]);

Ytrain=categorical(Ytrain);
Yvalidation=categorical(Yvalidation);
Ytest=categorical(Ytest);
%% CNN construction
% classes = [1 2 3 4 5];
% classWeights = [0.1 0.7 0.6 0.9 0.3];

classWeights = 1./countcats(Ytrain);
classWeights = classWeights'/mean(classWeights);

Layers=[
    imageInputLayer([height,width,channels]); %'DataAugmentation', 'none'); %'Normalization', 'none');
    
    convolution2dLayer([1 3], 256,'stride',[1 1], 'padding','same');  %Filter window size = [1 5], No of filters = 64, stride = [1 1];
    
    convolution2dLayer([1 3], 256,'stride',[1 1], 'padding','same');  
    reluLayer();
    dropoutLayer();
    maxPooling2dLayer([1 2],'stride',[1 2]); %PoolSize = [1 2], Stride = [1 1] 
    
    convolution2dLayer([1 3], 128,'stride',[1 1], 'padding','same');  
    reluLayer();
    
    convolution2dLayer([1 3], 128,'stride',[1 1], 'padding','same');  
    reluLayer();
    
    convolution2dLayer([1 3], 64,'stride',[1 1], 'padding','same');  
    reluLayer();
    dropoutLayer();
    maxPooling2dLayer([1 2],'stride',[1 2]); %PoolSize = [1 2], Stride = [1 1]
    
    
    fullyConnectedLayer(256); 
    dropoutLayer();
    fullyConnectedLayer(128);

    fullyConnectedLayer(5); %Reduce to five output classes
    softmaxLayer();
    classificationLayer();
    ];


%% Options of training
options = trainingOptions('sgdm','InitialLearnRate',0.001,'MaxEpochs',50, ...
    'LearnRateSchedule','piecewise', ...
    'LearnRateDropFactor',0.1, ...
    'LearnRateDropPeriod', 3,...
    'L2Regularizatio',1.0000e-04, ...
    'MiniBatchSize', 60,...
    'ValidationData',{Xvalidation, Yvalidation},...
    'Plots','training-progress');
convnet = trainNetwork(Xtrain,Ytrain,Layers,options);

Answer 1

[Aditya Patil]

As per my understanding, the data you have in unidimensional, and is time variant.

It might be a better option to consider this as a sequence classification problem. Have a look at this example for classifying ECG using LSTMs. There are also video tutorials for the same.

For the current model, it might be predicting the largest class for all the observations. You can check this by looking at the confusion matrix or the outputs.

[Ioana Cretu]

Thank you! I really wanted to use 1D CNN or at least to combine it with LSTMs. I modified the code so I can use the CNN features for LSTM but I have another error

Layers=[
    sequenceInputLayer(inputSize,'Normalization', 'zscore', 'Name','input'); 
    sequenceFoldingLayer('Name','fold')
    
    convolution2dLayer([1 7], 16,'stride',[1 1], 'padding','same','Name','conv1')
    batchNormalizationLayer('Name','bn1')
    maxPooling2dLayer([1 2],'stride',[1 2],'Name','mpool1')
    
    convolution2dLayer([1 7], 32,'stride',[1 1], 'padding','same','Name','conv2')
    batchNormalizationLayer('Name','bn2')
    reluLayer('Name','relu1')
    maxPooling2dLayer([1 2],'stride',[1 2],'Name','mpool2')
    
    convolution2dLayer([1 5], 64,'stride',[1 1], 'padding','same','Name','conv3') 
    batchNormalizationLayer('Name','bn3')
    reluLayer('Name','relu2')
    
    convolution2dLayer([1 5], 128,'stride',[1 1], 'padding','same','Name','conv4')
    batchNormalizationLayer('Name','bn4')
    reluLayer('Name','relu3')
    
    convolution2dLayer([1 3], 256,'stride',[1 1], 'padding','same','Name','conv5')  
    batchNormalizationLayer('Name','bn5')
    reluLayer('Name','relu4')
    maxPooling2dLayer([1 2],'stride',[1 2],'Name','mpool3')
    
    convolution2dLayer([1 3], 512,'stride',[1 1], 'padding','same','Name','conv6')
    batchNormalizationLayer('Name','bn6')
    reluLayer('Name','relu5')
    maxPooling2dLayer([1 2],'stride',[1 2],'Name','mpool4')
%     dropoutLayer('Name','dropout')
    
    sequenceUnfoldingLayer('Name','unfold')
    flattenLayer('Name','flatten')
    
    bilstmLayer(200,'Name','lstm')
    
    reluLayer('Name','relu6')
    fullyConnectedLayer(256,'Name','fc1')
    reluLayer('Name','relu7')
    fullyConnectedLayer(128,'Name','fc2')
    reluLayer('Name','relu8')
    fullyConnectedLayer(5,'Name','fc3')
    softmaxLayer('Name','softmax')
    classificationLayer('Name','classification')
    ];

lgraph = layerGraph(Layers);
lgraph = connectLayers(lgraph,'fold/miniBatchSize','unfold/miniBatchSize');

When I run this

convnet = trainNetwork(Xtrain,Ytrain,Layers,options);

I have this error

Error using trainNetwork (line 170)
Invalid network.

Caused by:
    Layer 'fold': Unconnected output. Each layer output must be connected to the input of another layer.
    Detected unconnected outputs:
        output 'miniBatchSize'
    Layer 'unfold': Unconnected input. Each layer input must be connected to the output of another layer.
    Detected unconnected inputs:
        input 'miniBatchSize'
        

Can you please give me an idea? What is the cause? I connected them using lgraph