Misclassification Rate in Machine Learning: Definition & Example

In machine learning, misclassification rate is a metric that tells us the percentage of observations that were incorrectly predicted by some classification model.

It is calculated as:

Misclassification Rate = # incorrect predictions / # total predictions

The value for misclassification rate can range from 0 to 1 where:

  • 0 represents a model that had zero incorrect predictions.
  • 1 represents a model that had completely incorrect predictions.

The lower the value for the misclassification rate, the better a classification model is able to predict the outcomes of the response variable.

The following example show how to calculate misclassification rate for a logistic regression model in practice.

Example: Calculating Misclassification Rate for a Logistic Regression Model

Suppose we use a logistic regression model to predict whether or not 400 different college basketball players get drafted into the NBA.

The following confusion matrix summarizes the predictions made by the model:

calculating misclassification rate of logistic regression model

Here is how to calculate the misclassification rate for the model:

  • Misclassification Rate = # incorrect predictions / # total predictions
  • Misclassification Rate = (false positive + false negative) / (total predictions)
  • Misclassification Rate = (70 + 40) / (400)
  • Misclassification Rate = 0.275

The misclassification rate for this model is 0.275 or 27.5%.

This means the model incorrectly predicted the outcome for 27.5% of the players.

The opposite of misclassification rate would be accuracy, which is calculated as:

  • Accuracy = 1 – Misclassification rate
  • Accuracy = 1 – 0.275
  • Accuracy = 0.725

This means the model correctly predicted the outcome for 72.5% of the players.

Pros & Cons of Misclassification Rate

Misclassification rate offers the following pros:

  • It’s easy to interpret. A misclassification rate of 10% means a model made an incorrect prediction for 10% of the total observations.
  • It’s easy to calculate. A misclassification rate is calculated as the number of total incorrect predictions divided by the total number of predictions.

However, misclassification rate has the following con:

  • It doesn’t take into account how the data is distributed. For example, suppose 90% of all players do not get drafted into the NBA. If we have a model that simply predicts every player to not get drafted, the model would have a misclassification rate of just 10%. This seems low, but, but the model is actually unable to correctly predict any player who gets drafted.

In practice, we often calculate the misclassification rate of a model along with other metrics like:

  • Sensitivity: The “true positive rate” – the percentage of positive outcomes the model is able to detect.
  • Specificity: The “true negative rate” – the percentage of negative outcomes the model is able to detect.
  • F1 Score: A metric that tells us the accuracy of a model, relative to how the data is distributed.

By calculating the value for each of these metrics, we can gain a full understanding of how well the model is able to make predictions.

Additional Resources

The following tutorials provide additional information about common machine learning concepts:

Introduction to Logistic Regression
What is Balanced Accuracy?
F1 Score vs. Accuracy: Which Should You Use?

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