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Using Classification Metrics

Input types

For the purposes of classification metrics, inputs (predictions and targets) are split into these categories (N stands for the batch size and C for number of classes):

*dtype binary means integers that are either 0 or 1

Type

preds shape

preds dtype

target shape

target dtype

Binary

(N,)

float

(N,)

binary*

Multi-class

(N,)

int

(N,)

int

Multi-class with logits or probabilities

(N, C)

float

(N,)

int

Multi-label

(N, …)

float

(N, …)

binary*

Multi-dimensional multi-class

(N, …)

int

(N, …)

int

Multi-dimensional multi-class with logits or probabilities

(N, C, …)

float

(N, …)

int

Note

All dimensions of size 1 (except N) are “squeezed out” at the beginning, so that, for example, a tensor of shape (N, 1) is treated as (N, ).

When predictions or targets are integers, it is assumed that class labels start at 0, i.e. the possible class labels are 0, 1, 2, 3, etc. Below are some examples of different input types

# Binary inputs
binary_preds  = torch.tensor([0.6, 0.1, 0.9])
binary_target = torch.tensor([1, 0, 2])

# Multi-class inputs
mc_preds  = torch.tensor([0, 2, 1])
mc_target = torch.tensor([0, 1, 2])

# Multi-class inputs with probabilities
mc_preds_probs  = torch.tensor([[0.8, 0.2, 0], [0.1, 0.2, 0.7], [0.3, 0.6, 0.1]])
mc_target_probs = torch.tensor([0, 1, 2])

# Multi-label inputs
ml_preds  = torch.tensor([[0.2, 0.8, 0.9], [0.5, 0.6, 0.1], [0.3, 0.1, 0.1]])
ml_target = torch.tensor([[0, 1, 1], [1, 0, 0], [0, 0, 0]])

Using the multiclass parameter

In some cases, you might have inputs which appear to be (multi-dimensional) multi-class but are actually binary/multi-label - for example, if both predictions and targets are integer (binary) tensors. Or it could be the other way around, you want to treat binary/multi-label inputs as 2-class (multi-dimensional) multi-class inputs.

For these cases, the metrics where this distinction would make a difference, expose the multiclass argument. Let’s see how this is used on the example of StatScores metric.

First, let’s consider the case with label predictions with 2 classes, which we want to treat as binary.

from torchmetrics.functional import stat_scores

# These inputs are supposed to be binary, but appear as multi-class
preds  = torch.tensor([0, 1, 0])
target = torch.tensor([1, 1, 0])

As you can see below, by default the inputs are treated as multi-class. We can set multiclass=False to treat the inputs as binary - which is the same as converting the predictions to float beforehand.

>>> stat_scores(preds, target, reduce='macro', num_classes=2)
tensor([[1, 1, 1, 0, 1],
        [1, 0, 1, 1, 2]])
>>> stat_scores(preds, target, reduce='macro', num_classes=1, multiclass=False)
tensor([[1, 0, 1, 1, 2]])
>>> stat_scores(preds.float(), target, reduce='macro', num_classes=1)
tensor([[1, 0, 1, 1, 2]])

Next, consider the opposite example: inputs are binary (as predictions are probabilities), but we would like to treat them as 2-class multi-class, to obtain the metric for both classes.

preds  = torch.tensor([0.2, 0.7, 0.3])
target = torch.tensor([1, 1, 0])

In this case we can set multiclass=True, to treat the inputs as multi-class.

>>> stat_scores(preds, target, reduce='macro', num_classes=1)
tensor([[1, 0, 1, 1, 2]])
>>> stat_scores(preds, target, reduce='macro', num_classes=2, multiclass=True)
tensor([[1, 1, 1, 0, 1],
        [1, 0, 1, 1, 2]])
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