Calculate Precision and Recall

machine-learning precision-recall
user1051536 picture user1051536 · Dec 8, 2012 · Viewed 19.9k times · Source

I am really confused about how to calculate Precision and Recall in Supervised machine learning algorithm using NB classifier

Say for example
1) I have two classes A,B
2) I have 10000 Documents out of which 2000 goes to training Sample set (class A=1000,class B=1000)
3) Now on basis of above training sample set classify rest 8000 documents using NB classifier
4) Now after classifying 5000 documents goes to class A and 3000 documents goes to class B
5) Now how to calculate Precision and Recall?

Please help me..

Thanks

Answer

Tom Marek picture Tom Marek · Dec 8, 2012

Hi you have to divide results into four groups -
True class A (TA) - correctly classified into class A
False class A (FA) - incorrectly classified into class A
True class B (TB) - correctly classified into class B
False class B (FB) - incorrectly classified into class B

precision = TA / (TA + FA)
recall = TA / (TA + FB)

You might also need accuracy and F-measure:

accuracy = (TA + TB) / (TA + TB + FA + FB)
f-measure = 2 * ((precision * recall)/(precision + recall))

More here:
http://en.wikipedia.org/wiki/Precision_and_recall#Definition_.28classification_context.29

Related questions

sklearn metrics for multiclass classification

I have performed GaussianNB classification using sklearn. I tried to calculate the metrics using the following code: print accuracy_score(y_test, y_pred) print precision_score(y_test, y_pred) Accuracy score is working correctly but precision score calculation …

Read More
What is a bad, decent, good, and excellent F1-measure range?

I understand F1-measure is a harmonic mean of precision and recall. But what values define how good/bad a F1-measure is? I can't seem to find any references (google or academic) answering my question.

Read More
What does recall mean in Machine Learning?

What's the meaning of recall of a classifier, e.g. bayes classifier? please give an example. for example, the Precision = correct/correct+wrong docs for test data. how to understand recall?

Read More