Consider that 20 stories of author X and 10 stories of author Y were kept together without mentioning the names of the authors. A classifier was then asked to predict the author (X or Y) of each of the stories. Later out of X’s stories 6 were classified as that of Y. On the other hand, out of Y’s stories 2 were classified as that of X. Considering X and Y as two classes, then which of the following is/are true?
Correct Answer :
Precision of class X is higher than the precision of class Y.
Accuracy of the classifier is 11/15.
Solution :
The correct options are:
1. Precision of class X is higher than the precision of class Y.
2. Accuracy of the classifier is 11/15.
Let us analyze the given problem step-by-step to understand why these options are correct.
First, we extract the given information from the problem statement:
Total stories of Author X (Actual Class X): 20
Total stories of Author Y (Actual Class Y): 10
Total number of stories: 20 + 10 = 30
Next, we look at the classifier's predictions:
- Out of X's 20 stories, 6 were classified as Y. This means:
- Stories of X correctly predicted as X (True Positives for X, or TPX): 20 - 6 = 14
- Stories of X incorrectly predicted as Y (False Negatives for X, or FNX): 6
- Out of Y's 10 stories, 2 were classified as X. This means:
- Stories of Y correctly predicted as Y (True Positives for Y, or TPY): 10 - 2 = 8
- Stories of Y incorrectly predicted as X (False Negatives for Y, or FNY): 2
From these classification results, we can construct the confusion matrix:
- True Class X predicted as X (TPX): 14
- True Class X predicted as Y (FNX / FPY): 6
- True Class Y predicted as X (FNY / FPX): 2
- True Class Y predicted as Y (TPY): 8
Now, let us calculate the performance metrics for both classes.
1. Accuracy:
Accuracy is the ratio of correctly predicted observations to the total observations.
Thus, the accuracy of the classifier is indeed 11/15, which matches one of the correct options.
2. Precision:
Precision is defined as the ratio of correctly predicted positive observations to the total predicted positive observations for a given class.
For class X:
Total predictions as X = TPX + FPX = 14 + 2 = 16
For class Y:
Total predictions as Y = TPY + FPY = 8 + 6 = 14
Comparing the precision values, we have 0.875 > 0.5714, which means the precision of class X is higher than the precision of class Y. This matches the other correct option.
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