Precision#

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

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, …).

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.

Examples

Binary case

metric = Precision(average=False)
metric.attach(default_evaluator, "precision")
y_true = torch.Tensor([1, 0, 1, 1, 0, 1])
y_pred = torch.Tensor([1, 0, 1, 0, 1, 1])
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["precision"])

0.75

Multiclass case

metric = Precision(average=False)
metric.attach(default_evaluator, "precision")
y_true = torch.Tensor([2, 0, 2, 1, 0, 1]).long()
y_pred = torch.Tensor([
    [0.0266, 0.1719, 0.3055],
    [0.6886, 0.3978, 0.8176],
    [0.9230, 0.0197, 0.8395],
    [0.1785, 0.2670, 0.6084],
    [0.8448, 0.7177, 0.7288],
    [0.7748, 0.9542, 0.8573],
])
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["precision"])

tensor([0.5000, 1.0000, 0.3333], dtype=torch.float64)

Precision can be computed as the unweighted average across all classes:

metric = Precision(average=True)
metric.attach(default_evaluator, "precision")
y_true = torch.Tensor([2, 0, 2, 1, 0, 1]).long()
y_pred = torch.Tensor([
    [0.0266, 0.1719, 0.3055],
    [0.6886, 0.3978, 0.8176],
    [0.9230, 0.0197, 0.8395],
    [0.1785, 0.2670, 0.6084],
    [0.8448, 0.7177, 0.7288],
    [0.7748, 0.9542, 0.8573],
])
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["precision"])

0.6111...

Multilabel case, the shapes must be (batch_size, num_categories, …)

metric = Precision(is_multilabel=True)
metric.attach(default_evaluator, "precision")
y_true = torch.Tensor([
    [0, 0, 1],
    [0, 0, 0],
    [0, 0, 0],
    [1, 0, 0],
    [0, 1, 1],
]).unsqueeze(0)
y_pred = torch.Tensor([
    [1, 1, 0],
    [1, 0, 1],
    [1, 0, 0],
    [1, 0, 1],
    [1, 1, 0],
]).unsqueeze(0)
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["precision"])

tensor([0.2000, 0.5000, 0.0000], dtype=torch.float64)

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

metric = Precision(average=False, output_transform=thresholded_output_transform)
metric.attach(default_evaluator, "precision")
y_true = torch.Tensor([1, 0, 1, 1, 0, 1])
y_pred = torch.Tensor([0.6, 0.2, 0.9, 0.4, 0.7, 0.65])
state = default_evaluator.run([[y_pred, y_true]])
print(state.metrics["precision"])

0.75

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.

Methods

update

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

update(output)[source]#

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