02/03/2020

# Accuracy, Precision, Recall, F1 Score and ROC curve

### Objectives

By the end of this post you should:

• understand why accuracy is not always the best metric of choice in classification tasks,
• understand the difference between accuracy, precision, recall and F1 score and be able to choose the right metric for your needs,
• be able to use Receiver Operating Characteristic (ROC) curve and Area Under the Curve (AUC) to make decisions about:
• which threshold (to classify a sample as positive) is better for your model,
• among all of the models you trained, which one is actually the best.

One of the most important decisions that have to be made before starting a Machine Learning project is to decide which metric to use. It is so crucial, in a sense that the wrong metric can potentially trick you to believe that your model is good, or getting better when in reality it is not.

I have a feeling that accuracy sometimes might be considered as “one metric to rule them all”. However, this is not the case. Choosing a metric should be done after asking a number of questions: What is trying to be achieved? What is the output of the model? What is the consequence of its decision? What about your data? What kind of it do you have? Is it balanced? These are some essential questions that one should consider before doing anything else.

This post introduces four metrics, namely: accuracy, precision, recall, and f1 score. None of these metrics are better, or worse than the other. The point here is to choose the right one for the problem you are trying to solve. Alright, let’s get started then… :)

To understand these metrics easier, I am going to use a tool called Confusion Matrix. It is simply a graph that shows us our model’s performance. For example, let’s say that our model classifies emotions based on a face image. Our confusion matrix could look something like this: Confusion Matrix

Columns of this graph show us how many samples a particular class has ( in this case, we have: happy=33, angry=29, sad=38 ), and rows show our model’s predictions. For example, we have 33 samples for the happy class, however, our model predicted happy=12, sad=15, angry=6. If we had all of the entries of this matrix on its diagonal, we would have a perfect model.

Now that we know how confusion matrix works, let’s come back to the main topic of this post. Imagine, our model’s predictions are as shown on the confusion matrix below, Another example of confusion matrix

Well, you need to be very careful now… If you didn’t think carefully before you choose the metric you used, and directly picked accuracy as your metric, you might fool yourself to believing that your model is amazing. Look, it has 99% accuracy! But… There are a number of problems here. The first, and probably the most obvious, one is: the data is imbalanced. Your model can easily learn to output a single label no matter what its input is and still have very high accuracy. There are different methods you can use to solve this particular problem, and one of them is to choose a different metric then accuracy ( since this post is not specifically about this problem, I don’t mention the other methods here ).

Another problem is: have you actually thought about how important is your model’s decision? What impact it has in real life? For example, what if your model predicts whether a person has cancer? or what if it predicts if a person is a terrorist? ( From the confusion matrix above ) That 2 times your model predicted wrong could potentially change somebody’s life drastically. In one of those cases, there is an individual who is not a terrorist, but your model says that s/he is. Furthermore, although your model’s accuracy is 99%, it wasn’t able to detect any terrorist, even though there is one. Or, if this was a model which is used to classify if a patient has an infectious virus, what do you think the consequence would be when our model says with 99% confidence that a person doesn’t have a virus when in reality s/he has?

To deal with these problems, let’s check some other metrics than accuracy.

## Precision

Let’s start with the mathematical formula of precision:

Let’s think about this equation a little bit. What benefit do we get using precision as our metric?

As long as there are no False Positives your model’s accuracy is $1$ (assuming True Positives $\geq$ 0). The moment you start having some False Positives, the precision starts decreasing. We had 99% accuracy on above example, let’s check what precision we have:

$$precision = \frac{0}{0+1} = 0.0$$

Now we are getting somewhere. So, based on precision, our model is not that good after all… But, when to use precision? If the number of False Positives are crucial to you, then you should use precision. For example, if your model predicts whether an email is a spam, you would be very concerned with the number of False Positives you have. Because a False Positive would mean that an email is spam, and maybe a user would miss a very important email because of that decision.

### When to use precision?

If the number of false positives your model makes is very crucial to you, and you don't mind a lot about false negatives, then precision could be the metric you should consider using.

## Recall

What about recall then? Again, let’s check the equation:

$$recall = \frac{True \ Positives}{True \ Positives + False \ Negatives}$$

This time, as long as there is no False Negative, we are home. Whenever the cost of having False Negatives is too high, maybe you should consider using recall as your metric. As an example, we can think of a model that predicts if a patient has an infectious virus. When we have a False Negative, we say, okay, this patient doesn’t have a virus, when in reality s/he has and the cost of having a False Positive is more people infected by the disease.

### When to use recall?

Recall punishes false negatives, hence it could be the metric of choice when the thing you care the most is false negatives your model make, and you don't worry too much about false positives.

## F1 Score

Before we delve into F1 Score’s equation, I would like you to imagine a situation where things can go wrong, let’s look at recall’s equation again:

$$recall=\frac{True \ Positives}{True \ Positives + False \ Negatives}$$

What could possibly go wrong here? Well, considering the classification problem we discussed earlier; what happens if we classify everyone as a terrorist? Let’s see:

$$recall = \frac{1}{1+0} = 1$$

As you can see, even though our model has very little value in reality (because we classify everybody as a terrorist!), it has a perfect recall score. Of course, there is a reverse proportion in between precision and recall, when we increase recall we decrease precision. But, do we need to keep track of both of these metrics when both are important to us? The answer is no because we have F1 Score to do that for us. Now let’s check its equation:

$$F1 = 2 * \frac{precision*recall}{precision+recall}$$

F1 score combines both precision and recall, so that our metric considers both of them. The reason why F1 score uses harmonic mean instead of averaging both values ( $\frac{precision+recall}{2}$ ) is because harmonic mean punishes extreme values. When we have   $precision=1.0$   and   $recall=0.0$   average of them is   $0.5$  , however, the harmonic mean is   $0.0$.

### When to use F1 Score?

When both precision and recall is important for you, you can consider using F1 Score as you metric.

## Receiver Operating Characteristic (ROC) Curve

ROC curve is a very useful visualization tool that can help us infer our model’s performance. It shows us the relationship between precision and recall as we vary a threshold for selecting positives. To be a bit more clear: if our model is trying to predict if a person is smiling from a given image, it could output a score between 0 and 43. We can then choose a threshold to decide when we have a positive (smile) sample. Like I said before, the ROC curve helps us to see our model’s performance with different thresholds.

In the ROC curve, we plot “False Positive Rate (FPR)” on the x-axis, and “True Positive Rate (TPR)” on the y-axis. We have already seen true positive rate in this post, it is recall. And the false positive rate is the probability of a false alarm: how many times our model wrongly classified an actual negative sample as positive?

$$True \ Positive \ Rate = \frac{True \ Positives}{True \ Positives + False \ Negatives}$$

$$False \ Positive \ Rate = \frac{False \ Positives}{False \ Positives + True \ Negatives}$$

The figure above shows an example of a ROC curve. Each curve drawn on the graph represents a model. When we move along a curve, we walk through different values of threshold. As it can be seen from the graph, each curve starts at the origin and ends at the right top corner of the graph. This is because of the extremes of the threshold value, namely 0 and 1. When the threshold is “0”, our model classifies everything as positive, so, both the True Positive Rate and False Positive Rate becomes 1 ( in true positive rate case we don’t have any “False Negatives”, and in false positive rate case we don’t have any “True Negatives” ). On the other hand, when the threshold is “1”, we classify everything as negative, so, both True Positive Rate, and False Positive Rate becomes 0. From this logic, we can see that the threshold decreases on a curve in the direction from top-right to bottom-left.

The black diagonal line shows the case when we have an equally true and false positive rate for each threshold value. This means that the model doesn’t have any ability to distinguish between positive and negative cases, it is a random model. We expect our trained model to draw a curve above the diagonal line.

To measure which model we train has a higher value to us, we use a method called ” Area Under the Curve” shortly AUC. The value of AUC also ranges between 0 and 1, and the higher is the better. As its name suggests, it is calculated by calculating the area under a drawn ROC curve. As an example, the AUC of the diagonal line is 0.5.

## Putting All Together – An Example

Let’s make an example to put everything we learned together. In this example, we want our model to classify if a person has an infectious virus. On its output, it gives us some score between $0.0-1.0$ (note that the score doesn’t need to be between $0.0-1.0$, it could be in another range) that we can use to predict if a patient is ill.

We have data of 1000 patients. This data contains some test results as well as the actual status of the patient (if s/he has the virus or not). Let’s say we trained a model, tested it on different thresholds, and the results are as shown in the table below:

Threshold TP FP TN FN
0.0 500 500 0 0
0.1 480 460 40 20
0.2 470 430 70 30
0.3 450 380 120 50
0.4 440 320 180 60
0.5 400 270 230 100
0.6 360 180 320 140
0.7 320 120 380 180
0.8 270 80 420 230
0.9 140 40 460 360
1.0 0 0 500 500

Note: Since it would take me more time to actually train a model and draw this table, I manually picked the values that are visible in the table. The point here is to understand how everything works, and I don’t think doing this causes any harm, but saves me some time :).

Based on this table, let’s calculate everything we learned so far. Because of each calculation shown below is exactly the same for each threshold, I am going to choose only 1 threshold for demonstration purposes, let’s say it is: $0.5$ Confusion matrix for threshold=0.5

$$precision = \frac{TP}{TP+FP} = \frac{400}{400+270} = 0.59$$

$$recall = \frac{TP}{TP+FN} = \frac{400}{400+100} = 0.8$$

$$F1 = 2*\frac{precision*recall}{precision+recall} = 2*\frac{0.59*0.8}{0.59+0.8} = 0.67$$

$$TPR = recall = 0.8$$

$$FPR = \frac{FP}{FP+TN} = \frac{270}{270+230} = 0.54$$ ROC curve for threshold=0.5 Calculations for each threshold value

Things to remember about ROC curve are as follows:

1. One curve describes one model. So, if we had different models, we would have as many curves as the models we have,
2. Moving along a curve gets us a model’s TPR and FPR values for different thresholds.
3. ROC curve helps us to see all of the concepts we learned in this post together.

When the threshold is 0.0 the model has perfect recall, however since it classifies everything as positive (so, everyone has the virus), the number of False Positives are very high, thus it’s precision is very low (hence the F1 Score is low). On the other hand, when the threshold is 1.0, the model predicts everything as negative (hence, nobody has the virus), so, both the model’s precision and recall is 0. When we move along the line and decrease the threshold, the model starts to classify more and more positives, hence recall starts increasing. However, together with True Positives, False Positives increases as well, this causes precision to decrease.

So in the end, we can play with different values of threshold and choose one that maximizes the metric of our choice. For example, if we want to use F1 Score as our metric, based on the given table above, we would choose 0.4 as our threshold, and use it whenever we want to make a prediction with our model.

## Recap

• Precision helps you to answer the following question: Among all of the positives my model predicted, how many percent of it was actually right? So when it is crucial for you to have a low amount of False Positives (e.g. if you classify an email as spam when your model outputs a positive), and “the negative cases (both false and true)” are less important for you, this could be the metric of your choice,
$$precision = \frac{True \ Positives}{True \ Positives + False \ Positives}$$
• Recall helps you to answer the following question: Among all of the actual positives I have, how many percent of it my model could actually predict right? So when it is crucial for you to have a low amount of False Negatives (e.g. a patient has an infectious virus and a false negative means that your model says that s/he doesn’t have).
$$recall = \frac{True \ Positives}{True \ Positives + False \ Negatives}$$
• F1 Score is the metric of choice when both precision and recall is important for you. Because this metric combines both together and punishes extreme values for each.
$$F1 = 2 * \frac{precision*recall}{precision+recall}$$
• Receiver Operating Characteristic (ROC) curve is obtained by plotting False Positive Rate (FPR) on the x-axis, and True Positive Rate (TPR) on the y-axis. A single curve represents a single model, and moving along it represents changing the threshold. ROC curve is very informative, in a sense that, it puts everything we learned in this post together and gives us a very nice visualization
$$True \ Positive \ Rate = \frac{True \ Positives}{True \ Positives + False \ Negatives}$$

$$False \ Positive \ Rate = \frac{False \ Positives}{False \ Positives + True \ Negatives}$$

• Area Under the Curve help you to answer the following question: Among all of the models I trained, which one is actually is the best? As its name suggests, it is calculated by calculating the area under the curve. In this blog post, we used ROC as our graph, so we calculated the area below the curves that were drawn on it.

## Conclusions

It is very crucial to understand that the metric you choose meters a lot. A wrongly chosen metric can fool you to think that the model you trained is much better than it is in reality. This, in return, potentially harm your business or maybe even worse.

You should keep in mind that accuracy is not the only metric that exists. While deciding which metric to use, you should ask yourself the following questions:

1. What my model is actually doing?
2. What is the consequence of my model’s decision?
3. What is the most important thing I want from my model? Is it a low number of False Positives? Low number of False Negatives? or both are important?
4. How do my data look like? Is it balanced?

After asking these questions, hopefully, you can come up with a metric that best suits your needs.

Please leave a comment below if you have any feedback, criticism, or something that you would like to discuss. I can also be reached on social media: @kivanc_yuksel
Tags: auc, classification, f1 score, false positive rate, machine learning, metrics, precision, recall, roc curve, true positive rate