I have a build a classification model using machine learning technique (SVM). I want to compare the classification accuracy of my model with a random classifier. My data set contains only two classes(1 or 0). The ratio of 1 and 0 instances are 35% and 65%. That means, 35% instances belong to 1 and 65% belong to 0 class. In that case, what will be the classification accuracy of random classifier (Random Guess)?


1 Answer 1


The equation of the classification accuracy for a random classifier (Random guess) is as follows:

Accuracy = 1/k (here k is the number of classes). In your case, the value of k is 2.

So, the classification accuracy of the random classifier in your case is 1/2 = 50%

  • $\begingroup$ Thank you very much for your support. Now, I understand it. $\endgroup$
    – Soikot Ali
    Nov 7, 2018 at 11:03
  • 2
    $\begingroup$ This is true if your random classifier gives even odds to all outcomes. But you could randomly choose 0 with probability .99 and 1 with probability .01 which would have an accuracy of 0.647 = 0.99*0.65 + 0.01*0.35 $\endgroup$
    – kbrose
    Nov 7, 2018 at 13:38
  • $\begingroup$ Thank you very much @kbrose. If my test dataset contains 20 instances (13 instances 0 and 7 instances 1). Then, what will be the accuracy of the random classier? Based on your explanation, I guess first I have to calculate accuracy for each instance. Second, sum up accuracy for all instances. Then calculate average accuracy by dividing 20. Am I understand correctly? $\endgroup$
    – Soikot Ali
    Nov 7, 2018 at 22:39

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