Metrics

sklego.metrics.correlation_score(column)

The correlation score can score how well the estimator predictions correlate with a given column.

This is especially useful to use in situations where "fairness" is a theme.

correlation_score takes a column on which to calculate the correlation and returns a metric function.

Parameters:

Name	Type	Description	Default
column	str | int	Name of the column (when X is a dataframe) or the index of the column (when X is a numpy array) to score against.	required

Returns:

Type	Description
Callable[..., float]	A function which calculates the negative correlation between estimator.predict(X) and X[column] (in gridsearch, larger is better and we want to typically punish correlation).

Examples:

from sklego.metrics import correlation_score
...
correlation_score('gender')(clf, X, y)

Source code in sklego/metrics.py

def correlation_score(column):
    """The correlation score can score how well the estimator predictions correlate with a given column.

    This is especially useful to use in situations where "fairness" is a theme.

    `correlation_score` takes a column on which to calculate the correlation and returns a metric function.

    Parameters
    ----------
    column : str | int
        Name of the column (when X is a dataframe) or the index of the column (when X is a numpy array) to score
        against.

    Returns
    -------
    Callable[..., float]
        A function which calculates the _negative_ correlation between `estimator.predict(X)` and `X[column]`
        (in gridsearch, larger is better and we want to typically punish correlation).

    Examples
    --------
    ```py
    from sklego.metrics import correlation_score
    ...
    correlation_score('gender')(clf, X, y)
    ```
    """

    def correlation_metric(estimator, X, y_true=None):
        """Remember: X is the thing going *in* to your pipeline."""
        sensitive_col = X[:, column] if isinstance(X, np.ndarray) else X[column]
        return -np.abs(np.corrcoef(estimator.predict(X), sensitive_col)[1, 0])

    return correlation_metric

sklego.metrics.equal_opportunity_score(sensitive_column, positive_target=1)

The equality opportunity score calculates the ratio between the probability of a true positive outcome given the sensitive attribute (column) being true and the same probability given the sensitive attribute being false.

\[\min \left(\frac{P(\hat{y}=1 | z=1, y=1)}{P(\hat{y}=1 | z=0, y=1)}, \frac{P(\hat{y}=1 | z=0, y=1)}{P(\hat{y}=1 | z=1, y=1)}\right)\]

This is especially useful to use in situations where "fairness" is a theme.

Parameters:

Name	Type	Description	Default
sensitive_column	str | int	Name of the column containing the binary sensitive attribute (when X is a dataframe) or the index of the column (when X is a numpy array).	required
positive_target		The name of the class which is associated with a positive outcome	1

Returns:

Type	Description
Callable[..., float]	A function which calculates the equal opportunity score for z = column

Examples:

from sklego.metrics import equal_opportunity_score
...
equal_opportunity_score('gender')(clf, X, y)

Source

M. Hardt, E. Price and N. Srebro (2016), Equality of Opportunity in Supervised Learning

Source code in sklego/metrics.py

def equal_opportunity_score(sensitive_column, positive_target=1):
    r"""The equality opportunity score calculates the ratio between the probability of a **true positive** outcome
    given the sensitive attribute (column) being true and the same probability given the sensitive attribute being
    false.

    $$\min \left(\frac{P(\hat{y}=1 | z=1, y=1)}{P(\hat{y}=1 | z=0, y=1)},
    \frac{P(\hat{y}=1 | z=0, y=1)}{P(\hat{y}=1 | z=1, y=1)}\right)$$

    This is especially useful to use in situations where "fairness" is a theme.

    Parameters
    ----------
    sensitive_column : str | int
        Name of the column containing the binary sensitive attribute (when X is a dataframe) or the index of the column
        (when X is a numpy array).
    positive_target: int, default=1
        The name of the class which is associated with a positive outcome

    Returns
    -------
    Callable[..., float]
        A function which calculates the equal opportunity score for z = column

    Examples
    --------
    ```py
    from sklego.metrics import equal_opportunity_score
    ...
    equal_opportunity_score('gender')(clf, X, y)
    ```

    Source
    ------
    M. Hardt, E. Price and N. Srebro (2016), Equality of Opportunity in Supervised Learning
    """

    def impl(estimator, X, y_true):
        """Remember: X is the thing going *in* to your pipeline."""
        sensitive_col = X[:, sensitive_column] if isinstance(X, np.ndarray) else X[sensitive_column]

        if not ((sensitive_col == 0) | (sensitive_col == 1)).all():
            raise ValueError(
                f"equal_opportunity_score only supports binary indicator columns for `column`. "
                f"Found values {np.unique(sensitive_col)}"
            )

        y_hat = estimator.predict(X)
        y_given_z1_y1 = y_hat[(sensitive_col == 1) & (y_true == positive_target)]
        y_given_z0_y1 = y_hat[(sensitive_col == 0) & (y_true == positive_target)]

        # If we never predict a positive target for one of the subgroups, the model is by definition not
        # fair so we return 0
        if len(y_given_z1_y1) == 0:
            warnings.warn(
                f"No samples with y_hat == {positive_target} for {sensitive_column} == 1, returning 0",
                RuntimeWarning,
            )
            return 0

        if len(y_given_z0_y1) == 0:
            warnings.warn(
                f"No samples with y_hat == {positive_target} for {sensitive_column} == 0, returning 0",
                RuntimeWarning,
            )
            return 0

        p_y1_z1 = np.mean(y_given_z1_y1 == positive_target)
        p_y1_z0 = np.mean(y_given_z0_y1 == positive_target)
        score = np.minimum(p_y1_z1 / p_y1_z0, p_y1_z0 / p_y1_z1)
        return score if not np.isnan(score) else 1

    return impl

sklego.metrics.p_percent_score(sensitive_column, positive_target=1)

The p_percent score calculates the ratio between the probability of a positive outcome given the sensitive attribute (column) being true and the same probability given the sensitive attribute being false.

\[\min \left(\frac{P(\hat{y}=1 | z=1)}{P(\hat{y}=1 | z=0)}, \frac{P(\hat{y}=1 | z=0)}{P(\hat{y}=1 | z=1)}\right)\]

This is especially useful to use in situations where "fairness" is a theme.

Parameters:

Name	Type	Description	Default
sensitive_column	str | int	Name of the column containing the binary sensitive attribute (when X is a dataframe) or the index of the column (when X is a numpy array).	required
positive_target	int	The name of the class which is associated with a positive outcome	1

Returns:

Type	Description
Callable[..., float]	A function which calculates the p percent score for z = column

Examples:

from sklego.metrics import p_percent_score
...
p_percent_score('gender')(clf, X, y)

Source

M. Zafar et al. (2017), Fairness Constraints: Mechanisms for Fair Classification

Source code in sklego/metrics.py

def p_percent_score(sensitive_column, positive_target=1):
    r"""The p_percent score calculates the ratio between the probability of a positive outcome given the sensitive
    attribute (column) being true and the same probability given the sensitive attribute being false.

    $$\min \left(\frac{P(\hat{y}=1 | z=1)}{P(\hat{y}=1 | z=0)}, \frac{P(\hat{y}=1 | z=0)}{P(\hat{y}=1 | z=1)}\right)$$

    This is especially useful to use in situations where "fairness" is a theme.

    Parameters
    ----------
    sensitive_column : str | int
        Name of the column containing the binary sensitive attribute (when X is a dataframe) or the index of the column
        (when X is a numpy array).
    positive_target : int, default=1
        The name of the class which is associated with a positive outcome

    Returns
    -------
    Callable[..., float]
        A function which calculates the p percent score for z = column

    Examples
    --------
    ```py
    from sklego.metrics import p_percent_score
    ...
    p_percent_score('gender')(clf, X, y)
    ```

    Source
    ------
    M. Zafar et al. (2017), Fairness Constraints: Mechanisms for Fair Classification
    """

    def impl(estimator, X, y_true=None):
        """Remember: X is the thing going *in* to your pipeline."""
        sensitive_col = X[:, sensitive_column] if isinstance(X, np.ndarray) else X[sensitive_column]

        if not ((sensitive_col == 0) | (sensitive_col == 1)).all():
            raise ValueError(
                f"p_percent_score only supports binary indicator columns for `column`. "
                f"Found values {np.unique(sensitive_col)}"
            )

        y_hat = estimator.predict(X)
        y_given_z1 = y_hat[sensitive_col == 1]
        y_given_z0 = y_hat[sensitive_col == 0]
        p_y1_z1 = np.mean(y_given_z1 == positive_target)
        p_y1_z0 = np.mean(y_given_z0 == positive_target)

        # If we never predict a positive target for one of the subgroups, the model is by definition not
        # fair so we return 0
        if p_y1_z1 == 0:
            warnings.warn(
                f"No samples with y_hat == {positive_target} for {sensitive_column} == 1, returning 0",
                RuntimeWarning,
            )
            return 0

        if p_y1_z0 == 0:
            warnings.warn(
                f"No samples with y_hat == {positive_target} for {sensitive_column} == 0, returning 0",
                RuntimeWarning,
            )
            return 0

        p_percent = np.minimum(p_y1_z1 / p_y1_z0, p_y1_z0 / p_y1_z1)
        return p_percent if not np.isnan(p_percent) else 1

    return impl

sklego.metrics.subset_score(subset_picker, score, **kwargs)

Return a method that applies the passed score only to a specific subset.

The subset picker is a method that is passed the corresponding X and y_true and returns a one-dimensional boolean vector where every element corresponds to a row in the data.

Only the elements with a True value are taken into account for the passed score, representing a filter.

This allows users to have an easy approach to measuring metrics over different slices of the population which can give insights into the model performance, either specifically for fairness or in general.

Parameters:

Name	Type	Description	Default
subset_picker	Callable	Method that returns a boolean mask that is used for slicing the samples	required
score	Callable[..., T]	The score that needs to be applied to the subset	required
kwargs	dict	Additional keyword arguments to pass to score	{}

Returns:

Type	Description
Callable[..., T]	A function which calculates the passed score for the subset

Examples:

from sklego.metrics import subset_score
...
subset_score(lambda X, y_true: X['column'] == 'A', accuracy_score)(clf, X, y)

Source code in sklego/metrics.py

def subset_score(subset_picker: Callable, score: Callable, **kwargs):
    """Return a method that applies the passed score only to a specific subset.

    The subset picker is a method that is passed the corresponding `X` and `y_true` and returns a one-dimensional
    boolean vector where every element corresponds to a row in the data.

    Only the elements with a True value are taken into account for the passed score, representing a filter.

    This allows users to have an easy approach to measuring metrics over different slices of the population which can
    give insights into the model performance, either specifically for fairness or in general.

    Parameters
    ----------
    subset_picker : Callable
        Method that returns a boolean mask that is used for slicing the samples
    score : Callable[..., T]
        The score that needs to be applied to the subset
    kwargs : dict
        Additional keyword arguments to pass to score

    Returns
    -------
    Callable[..., T]
        A function which calculates the passed score for the subset

    Examples
    --------
    ```py
    from sklego.metrics import subset_score
    ...
    subset_score(lambda X, y_true: X['column'] == 'A', accuracy_score)(clf, X, y)
    ```
    """

    def sliced_metric(estimator, X, y_true=None):
        mask = subset_picker(X, y_true)
        if isinstance(mask, np.ndarray):
            if len(mask.shape) > 1:
                raise ValueError(
                    "`subset_picker` should return 1-dimensional numpy array or Pandas"
                    + " series, returned {} instead".format(len(mask.shape))
                )
        if np.sum(mask) == 0:
            warnings.warn("No samples in subset, returning NaN", RuntimeWarning)
            return np.nan
        X = X[mask]
        y_pred = estimator.predict(X)
        return score(y_true[mask], y_pred, **kwargs)

    return sliced_metric