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ML | MultiLabel Ranking Metrics – Coverage Error
  • Last Updated : 23 Mar, 2020

The coverage error tells us how many top-scored final prediction labels we have to include without missing any ground truth label. This is useful if we want to know the average number of top-scored-prediction required to predict in order to not miss any ground truth label.
Given a binary indicator matrix of ground-truth labels y\epsilon \left \{ 0, 1 \right \}^{n_{samples} * n_{labels}}. The score associated with each label is denoted by \hat{f} where,


 \hat{f}\epsilon \left \{ \mathbb{R} \right \}^{n_{samples} * n_{labels}}

The coverage error is defined as:

 coverage\left ( y, \hat{f} \right ) = \dfrac{1}{n_{samples}} * \sum_{i=0}^{n_{samples}-1}max_{j: y_{ij}=1} rank_{ij}

where rank is defined as

 rank_{ij} = \left | \left \{  k \colon \hat{f_{ik}}\geq\hat{f_{ij}} \right \}\right |

Code: To check for coverage Error for any prediction scores with true-labels using scikit-learn.

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# Import dataset
import numpy as np
from sklearn.metrics import coverage_error
  
# Create Imaginary prediction and truth dataset
y_true = np.array([[1, 0, 1], [0, 0, 1], [0, 1, 1]])
y_pred_score = np.array([[0.75, 0.5, 1], [1, 1, 1.2], [2.3, 1.2, 0.1]])
print(coverage_error(y_true, y_pred_score))

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Output:

coverage error of 2.0

Let’s calculate the coverage error of above example manually
Our first sample has ground-truth value of [1, 0, 1]. To cover both true labels we need to look our predictions (here [0.75, 0.5, 1]) into descending order. Thus, we need top-2 predicted labels in this sample. Similarly for second and third samples, we need top-1 and top-2 predicted samples. Averaging these results over a number of samples gives us an output of 2.0.


  Coverage Error  =\dfrac{\left ( 2+1+3 \right )}{3} = 2.0

The best value of coverage is when it is equal to average number of true class labels.

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