python - Keras: Classification report accuracy is different between model.predict accuracy for multiclass

Question

Colab link is here:

The data is imported the following was

train_ds = tf.keras.preprocessing.image_dataset_from_directory(
    main_folder,
    validation_split=0.1,
    subset="training",
    label_mode='categorical',
    seed=123,
    image_size=(dim, dim))

val_ds = tf.keras.preprocessing.image_dataset_from_directory(
    main_folder,
    validation_split=0.1,
    subset="validation",
    label_mode='categorical',
    seed=123,
    image_size=(dim, dim))

The model is trained the following way

model = tf.keras.models.Sequential([
    tf.keras.layers.experimental.preprocessing.Rescaling(1. / 255),
    ...
    tf.keras.layers.Dense(2, activation='softmax')
])

model.compile(optimizer="adam", loss=tf.keras.losses.CategoricalCrossentropy(), metrics=['accuracy'])

I am struggling with getting the right predicted categories and right true_categories to get the classification report to work:

y_pred = model.predict(val_ds, batch_size=1)
predicted_categories = np.argmax(y_pred, axis=1)

true_categories = tf.concat([y for x, y in val_ds], axis=0).numpy()
true_categories_argmax = np.argmax(true_categories, axis=1)

print(classification_report(true_categories_argmax, predicted_categories))

At the moment the output of the epoch is contradicting the classification report

Epoch 22/75
144/144 [==============================] - 7s 48ms/step - loss: 0.0611 - accuracy: 0.9776 - val_loss: 0.0768 - val_accuracy: 0.9765

The validation set on the model returns

model.evaluate(val_ds)

[==============================] - 0s 16ms/step - loss: 0.0696 - accuracy: 0.9784
[0.06963862478733063, 0.9784313440322876]

while the classification report is very different:

          precision    recall  f1-score   support
     0.0       0.42      0.44      0.43       221
     1.0       0.56      0.54      0.55       289
    accuracy                           0.49       510
   macro avg       0.49      0.49      0.49       510
weighted avg       0.50      0.49      0.50       510

Similiar questions here, here, here, here, here with no answers to this issue.

Answer 1

You set label_mode='categorical' then this is a multi-class classification and you need to use softmax activation in your last dense layer. Because softmax force the outputs sum to be equal to 1. You can kinda interpret them as probabilities. With sigmoid it will not be possible to find the dominant class. It can assign any values without restriction.

My model's last layer: Dense(5, activation = 'softmax')

My model's loss: loss=tf.keras.losses.CategoricalCrossentropy(), same as yours. Labels are one-hot-encoded in this case.

Explanation: I used a 5 class classification for demo purposes, but it follows the same logic.

y_pred = model.predict(val_ds)

y_pred[:2]
>>> array([[0.28257513, 0.4343998 , 0.18222839, 0.04164065, 0.05915598],
       [0.36404607, 0.08850227, 0.15335019, 0.21602921, 0.17807229]],
      dtype=float32)

This incidates each classes probabilities, for example first example has a probability of 43% being belong to class 2. You need to use argmax to find class index.

predicted_categories = np.argmax(y_pred, axis = 1)
predicted_categories[:2]

array([1, 0])

We now have the predicted classes. Now need to obtain true classes.

true_categories = tf.concat([y for x, y in val_ds], axis = 0).numpy() # convert to np array

    true_categories[:2]
>>> array([[1., 0., 0., 0., 0.],
       [0., 0., 0., 0., 1.]], dtype=float32)

If you feed this into classification report, you will get following:

ValueError: Classification metrics can't handle a mix of multilabel-indicator and multiclass targets

We need to also do:

    true_categories_argmax = np.argmax(true_categories, axis = 1)
    true_categories_argmax[:2]
>>> array([0, 4])

Now it is ready to for comparison.

print(classification_report(true_categories_argmax, predicted_categories))

That should produce the expected result:

      precision    recall  f1-score   support

   0       0.55      0.43      0.48       129
   1       0.53      0.83      0.64       176
   2       0.48      0.56      0.52       120
   3       0.75      0.72      0.73       152
   4       0.66      0.31      0.42       157

Edit: Classes might get shuffled as tf.keras.preprocessing.image_dataset_from_directory sets shuffle = True. For val_ds try to set shuffle = False. Like this:

val_ds = tf.keras.preprocessing.image_dataset_from_directory(
    main_folder,
    validation_split=0.1,
    subset="validation",
    shuffle = False,
    label_mode='categorical',
    seed=123,
    image_size=(dim, dim))

Edit2: Here is what I came up with:

prediction_classes = np.array([])
true_classes =  np.array([])

for x, y in val_ds:
  prediction_classes = np.concatenate([prediction_classes,
                       np.argmax(model.predict(x), axis = -1)])
  true_classes = np.concatenate([true_classes, np.argmax(y.numpy(), axis=-1)])

Classification Report:

print(classification_report(true_classes, prediction_classes))

              precision    recall  f1-score   support

         0.0       0.74      0.81      0.77      1162
         1.0       0.80      0.72      0.75      1179

    accuracy                           0.77      2341
   macro avg       0.77      0.77      0.76      2341
weighted avg       0.77      0.77      0.76      2341