How does Keras calculate accuracy?

Question

How does Keras calculate accuracy from the classwise probabilities? Say, for example we have 100 samples in the test set which can belong to one of two classes. We also have a list of the classwise probabilites. What threshold does Keras use to assign a sample to either of the two classes?

Answer 1

For binary classification, the code for accuracy metric is:

K.mean(K.equal(y_true, K.round(y_pred)))

which suggests that 0.5 is the threshold to distinguish between classes. y_true should of course be 1-hots in this case.

It's a bit different for categorical classification:

K.mean(K.equal(K.argmax(y_true, axis=-1), K.argmax(y_pred, axis=-1)))

which means "how often predictions have maximum in the same spot as true values"

There is also an option for top-k categorical accuracy, which is similar to one above, but calculates how often target class is within the top-k predictions.