Useful metrics for training

Training metrics

Metrics for training fastai models are simply functions that take input and target tensors, and return some metric of interest for training. You can write your own metrics by defining a function of that type, and passing it to Learner in the metrics parameter, or use one of the following pre-defined functions.

Predefined metrics:

accuracy[source][test]

accuracy(input:Tensor, targs:Tensor) → Rank0Tensor

Tests found for accuracy:

  • pytest -sv tests/test_metrics.py::test_accuracy [source]
  • pytest -sv tests/test_vision_train.py::test_accuracy [source]

To run tests please refer to this guide.

Compute accuracy with targs when input is bs * n_classes.

accuracy_thresh[source][test]

accuracy_thresh(y_pred:Tensor, y_true:Tensor, thresh:float=0.5, sigmoid:bool=True) → Rank0Tensor

Tests found for accuracy_thresh:

  • pytest -sv tests/test_metrics.py::test_accuracy_thresh [source]

To run tests please refer to this guide.

Compute accuracy when y_pred and y_true are the same size.

Prediction are compared to thresh after sigmoid is maybe applied. Then we count the numbers that match the targets.

top_k_accuracy[source][test]

top_k_accuracy(input:Tensor, targs:Tensor, k:int=5) → Rank0Tensor

Tests found for top_k_accuracy:

  • pytest -sv tests/test_metrics.py::test_top_k_accuracy [source]

To run tests please refer to this guide.

Computes the Top-k accuracy (target is in the top k predictions).

dice[source][test]

dice(input:Tensor, targs:Tensor, iou:bool=False) → Rank0Tensor

Tests found for dice:

  • pytest -sv tests/test_metrics.py::test_dice [source]
  • pytest -sv tests/test_metrics.py::test_dice_iou [source]

To run tests please refer to this guide.

Dice coefficient metric for binary target. If iou=True, returns iou metric, classic for segmentation problems.

error_rate[source][test]

error_rate(input:Tensor, targs:Tensor) → Rank0Tensor

Tests found for error_rate:

  • pytest -sv tests/test_metrics.py::test_error_rate [source]
  • pytest -sv tests/test_vision_train.py::test_error_rate [source]

To run tests please refer to this guide.

1 - accuracy

mean_squared_error[source][test]

mean_squared_error(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for mean_squared_error. To contribute a test please refer to this guide and this discussion.

Mean squared error between pred and targ.

mean_absolute_error[source][test]

mean_absolute_error(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for mean_absolute_error. To contribute a test please refer to this guide and this discussion.

Mean absolute error between pred and targ.

mean_squared_logarithmic_error[source][test]

mean_squared_logarithmic_error(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for mean_squared_logarithmic_error. To contribute a test please refer to this guide and this discussion.

Mean squared logarithmic error between pred and targ.

exp_rmspe[source][test]

exp_rmspe(pred:Tensor, targ:Tensor) → Rank0Tensor

Tests found for exp_rmspe:

  • pytest -sv tests/test_metrics.py::test_exp_rmspe [source]
  • pytest -sv tests/test_metrics.py::test_exp_rmspe_num_of_ele [source]

To run tests please refer to this guide.

Exp RMSE between pred and targ.

root_mean_squared_error[source][test]

root_mean_squared_error(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for root_mean_squared_error. To contribute a test please refer to this guide and this discussion.

Root mean squared error between pred and targ.

fbeta[source][test]

fbeta(y_pred:Tensor, y_true:Tensor, thresh:float=0.2, beta:float=2, eps:float=1e-09, sigmoid:bool=True) → Rank0Tensor

Tests found for fbeta:

  • pytest -sv tests/test_metrics.py::test_fbeta [source]

To run tests please refer to this guide.

Computes the f_beta between preds and targets

beta determines the value of the fbeta applied, eps is there for numeric stability. If sigmoid=True, a sigmoid is applied to the predictions before comparing them to thresh then to the targets. See the F1 score wikipedia page for details on the fbeta score.

explained_variance[source][test]

explained_variance(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for explained_variance. To contribute a test please refer to this guide and this discussion.

Explained variance between pred and targ.

r2_score[source][test]

r2_score(pred:Tensor, targ:Tensor) → Rank0Tensor

No tests found for r2_score. To contribute a test please refer to this guide and this discussion.

R2 score (coefficient of determination) between pred and targ.

The following metrics are classes, don't forget to instantiate them when you pass them to a Learner.

class RMSE[source][test]

RMSE() :: RegMetrics

No tests found for RMSE. To contribute a test please refer to this guide and this discussion.

Compute the root mean squared error.

class ExpRMSPE[source][test]

ExpRMSPE() :: RegMetrics

No tests found for ExpRMSPE. To contribute a test please refer to this guide and this discussion.

Compute the exponential of the root mean square error.

class Precision[source][test]

Precision(average:Optional[str]='binary', pos_label:int=1, eps:float=1e-09) :: CMScores

No tests found for Precision. To contribute a test please refer to this guide and this discussion.

Compute the Precision.

class Recall[source][test]

Recall(average:Optional[str]='binary', pos_label:int=1, eps:float=1e-09) :: CMScores

No tests found for Recall. To contribute a test please refer to this guide and this discussion.

Compute the Recall.

class FBeta[source][test]

FBeta(average:Optional[str]='binary', pos_label:int=1, eps:float=1e-09, beta:float=2) :: CMScores

No tests found for FBeta. To contribute a test please refer to this guide and this discussion.

Compute the Fbeta score.

class R2Score[source][test]

R2Score() :: RegMetrics

No tests found for R2Score. To contribute a test please refer to this guide and this discussion.

Compute the R2 score (coefficient of determination).

class ExplainedVariance[source][test]

ExplainedVariance() :: RegMetrics

No tests found for ExplainedVariance. To contribute a test please refer to this guide and this discussion.

Compute the explained variance.

class MatthewsCorreff[source][test]

MatthewsCorreff() :: ConfusionMatrix

No tests found for MatthewsCorreff. To contribute a test please refer to this guide and this discussion.

Compute the Matthews correlation coefficient.

class KappaScore[source][test]

KappaScore(weights:Optional[str]=None) :: ConfusionMatrix

No tests found for KappaScore. To contribute a test please refer to this guide and this discussion.

Compute the rate of agreement (Cohens Kappa).

Ref.: https://github.com/scikit-learn/scikit-learn/blob/bac89c2/sklearn/metrics/classification.py

KappaScore supports linear and quadratic weights on the off-diagonal cells in the ConfusionMatrix, in addition to the default unweighted calculation treating all misclassifications as equally weighted. Leaving KappaScore's weights attribute as None returns the unweighted Kappa score. Updating weights to "linear" means off-diagonal ConfusionMatrix elements are weighted in linear proportion to their distance from the diagonal; "quadratic" means weights are squared proportional to their distance from the diagonal. Specify linear or quadratic weights, if using, by first creating an instance of the metric and then updating the weights attribute, similar to as follows:

kappa = KappaScore()
kappa.weights = "quadratic"
learn = cnn_learner(data, model, metrics=[error_rate, kappa])

class ConfusionMatrix[source][test]

ConfusionMatrix() :: Callback

No tests found for ConfusionMatrix. To contribute a test please refer to this guide and this discussion.

Computes the confusion matrix.

Creating your own metric

Creating a new metric can be as simple as creating a new function. If your metric is an average over the total number of elements in your dataset, just write the function that will compute it on a batch (taking pred and targ as arguments). It will then be automatically averaged over the batches (taking their different sizes into account).

Sometimes metrics aren't simple averages however. If we take the example of precision for instance, we have to divide the number of true positives by the number of predictions we made for that class. This isn't an average over the number of elements we have in the dataset, we only consider those where we made a positive prediction for a specific thing. Computing the precision for each batch, then averaging them will yield to a result that may be close to the real value, but won't be it exactly (and it really depends on how you deal with special case of 0 positive predictions).

This why in fastai, every metric is implemented as a callback. If you pass a regular function, the library transforms it to a proper callback called AverageCallback. The callback metrics are only called during the validation phase, and only for the following events:

  • on_epoch_begin (for initialization)
  • on_batch_begin (if we need to have a look at the input/target and maybe modify them)
  • on_batch_end (to analyze the last results and update our computation)
  • on_epoch_end(to wrap up the final result that should be added to last_metrics)

As an example, the following code is the exact implementation of the AverageMetric callback that transforms a function like accuracy into a metric callback.

class AverageMetric(Callback):
    "Wrap a `func` in a callback for metrics computation."
    def __init__(self, func):
        # If it's a partial, use func.func
        name = getattr(func,'func',func).__name__
        self.func, self.name = func, name

    def on_epoch_begin(self, **kwargs):
        "Set the inner value to 0."
        self.val, self.count = 0.,0

    def on_batch_end(self, last_output, last_target, **kwargs):
        "Update metric computation with `last_output` and `last_target`."
        if not is_listy(last_target): last_target=[last_target]
        self.count += last_target[0].size(0)
        val = self.func(last_output, *last_target)
        self.val += last_target[0].size(0) * val.detach().cpu()

    def on_epoch_end(self, last_metrics, **kwargs):
        "Set the final result in `last_metrics`."
        return add_metrics(last_metrics, self.val/self.count)

Here add_metrics is a convenience function that will return the proper dictionary for us:

{'last_metrics': last_metrics + [self.val/self.count]}

And here is another example that properly computes the precision for a given class.

class Precision(Callback):
    
    def on_epoch_begin(self, **kwargs):
        self.correct, self.total = 0, 0
    
    def on_batch_end(self, last_output, last_target, **kwargs):
        preds = last_output.argmax(1)
        self.correct += ((preds==0) * (last_target==0)).float().sum()
        self.total += (preds==0).float().sum()
    
    def on_epoch_end(self, last_metrics, **kwargs):
        return add_metrics(last_metrics, self.correct/self.total)

The following custom callback class example measures peak RAM usage during each epoch:

import tracemalloc
class TraceMallocMetric(Callback):
    def __init__(self):
        super().__init__()
        self.name = "peak RAM"

    def on_epoch_begin(self, **kwargs):
        tracemalloc.start()
        
    def on_epoch_end(self, last_metrics, **kwargs):
        current, peak =  tracemalloc.get_traced_memory()
        tracemalloc.stop()
        return add_metrics(last_metrics, torch.tensor(peak))

To deploy it, you need to pass an instance of this custom metric in the metrics argument:

learn = cnn_learner(data, model, metrics=[accuracy, TraceMallocMetric()])
learn.fit_one_cycle(3, max_lr=1e-2)

And then the output changes to:

Total time: 00:54
epoch   train_loss  valid_loss  accuracy    peak RAM
   1    0.333352    0.084342    0.973800    2395541.000000
   2    0.096196    0.038386    0.988300    2342145.000000
   3    0.048722    0.029234    0.990200    2342680.000000

As mentioner earlier, using the metrics argument with a custom metrics class is limited in the number of phases of the callback system it can access, it can only return one numerical value and as you can see its output is hardcoded to have 6 points of precision in the output, even if the number is an int.

To overcome these limitations callback classes should be used instead.

For example, the following class:

  • uses phases not available for the metric classes
  • it reports 3 columns, instead of just one
  • its column report ints, instead of floats
import tracemalloc
class TraceMallocMultiColMetric(LearnerCallback):
    _order=-20 # Needs to run before the recorder
    def __init__(self, learn):
        super().__init__(learn)
        self.train_max = 0

    def on_train_begin(self, **kwargs):
        self.learn.recorder.add_metric_names(['used', 'max_used', 'peak'])
            
    def on_batch_end(self, train, **kwargs):
        # track max memory usage during the train phase
        if train:
            current, peak =  tracemalloc.get_traced_memory()
            self.train_max = max(self.train_max, current)
        
    def on_epoch_begin(self, **kwargs):
        tracemalloc.start()

    def on_epoch_end(self, last_metrics, **kwargs):
        current, peak =  tracemalloc.get_traced_memory()
        tracemalloc.stop()
        return add_metrics(last_metrics, [current, self.train_max, peak])

Note, that it subclasses LearnerCallback and not Callback, since the former provides extra features not available in the latter.

Also _order=-20 is crucial - without it the custom columns will not be added - it tells the callback system to run this callback before the recorder system.

To deploy it, you need to pass the name of the class (not an instance!) of the class in the callback_fns argument. This is because the learn object doesn't exist yet, and it's required to instantiate TraceMallocMultiColMetric. The system will do it for us automatically as soon as the learn object has been created.

learn = cnn_learner(data, model, metrics=[accuracy], callback_fns=TraceMallocMultiColMetric)
learn.fit_one_cycle(3, max_lr=1e-2)

And then the output changes to:

Total time: 00:53
epoch   train_loss valid_loss   accuracy     used   max_used   peak
    1   0.321233    0.068252    0.978600    156504  2408404   2419891 
    2   0.093551    0.032776    0.988500     79343  2408404   2348085
    3   0.047178    0.025307    0.992100     79568  2408404   2342754

Another way to do the same is by using learn.callbacks.append, and this time we need to instantiate TraceMallocMultiColMetric with learn object which we now have, as it is called after the latter was created:

learn = cnn_learner(data, model, metrics=[accuracy])
learn.callbacks.append(TraceMallocMultiColMetric(learn))
learn.fit_one_cycle(3, max_lr=1e-2)

Configuring the custom metrics in the learn object sets them to run in all future fit-family calls. However, if you'd like to configure it for just one call, you can configure it directly inside fit or fit_one_cycle:

learn = cnn_learner(data, model, metrics=[accuracy])
learn.fit_one_cycle(3, max_lr=1e-2, callbacks=TraceMallocMultiColMetric(learn))

And to stress the differences:

  • the callback_fns argument expects a classname or a list of those
  • the callbacks argument expects an instance of a class or a list of those
  • learn.callbacks.append expects a single instance of a class

For more examples, look inside fastai codebase and its test suite, search for classes that subclass either Callback, LearnerCallback and subclasses of those two.

Finally, while the above examples all add to the metrics, it's not a requirement. A callback can do anything it wants and it is not required to add its outcomes to the metrics printout.