Precision

class ignite.metrics.precision.Precision(output_transform=<function Precision.<lambda>>, average=False, is_multilabel=False, device=device(type='cpu'))

Calculates precision for binary and multiclass data.

$$\text{Precision} = \frac{ TP }{ TP + FP }$$

where $\text{TP}$ is true positives and $\text{FP}$ is false positives.

• update must receive output of the form (y_pred, y) or {'y_pred': y_pred, 'y': y}.

• y_pred must be in the following shape (batch_size, num_categories, …) or (batch_size, …).

• y must be in the following shape (batch_size, …).

In binary and multilabel cases, the elements of y and y_pred should have 0 or 1 values. Thresholding of predictions can be done as below:

def thresholded_output_transform(output):
    y_pred, y = output
    y_pred = torch.round(y_pred)
    return y_pred, y

precision = Precision(output_transform=thresholded_output_transform)

In multilabel cases, average parameter should be True. However, if user would like to compute F1 metric, for example, average parameter should be False. This can be done as shown below:

precision = Precision(average=False)
recall = Recall(average=False)
F1 = precision * recall * 2 / (precision + recall + 1e-20)
F1 = MetricsLambda(lambda t: torch.mean(t).item(), F1)

Warning

In multilabel cases, if average is False, current implementation stores all input data (output and target) in as tensors before computing a metric. This can potentially lead to a memory error if the input data is larger than available RAM.

Parameters

• output_transform (Callable) – a callable that is used to transform the Engine’s process_function’s output into the form expected by the metric. This can be useful if, for example, you have a multi-output model and you want to compute the metric with respect to one of the outputs.

• average (bool) – if True, precision is computed as the unweighted average (across all classes in multiclass case), otherwise, returns a tensor with the precision (for each class in multiclass case).

• is_multilabel (bool) – flag to use in multilabel case. By default, value is False. If True, average parameter should be True and the average is computed across samples, instead of classes.

• device (Union[str, device]) – specifies which device updates are accumulated on. Setting the metric’s device to be the same as your update arguments ensures the update method is non-blocking. By default, CPU.

Methods

update

Updates the metric's state using the passed batch output.

update(output)

Updates the metric’s state using the passed batch output.

By default, this is called once for each batch.

Parameters

output (Sequence[Tensor]) – the is the output from the engine’s process function.

Return type

None