Assume that I have a binary classification problem. The outcome from classification I am mostly interested in is the well-calibrated probabilities.

The first way to check this is the calibration plot (or reliability curve).

The question: is it fair enough to judge about calibration based on Brier score?

Assume that we have "enough" data. Would the classifier with smaller Brier score provide a rather better reliability curve?

My concern comes from the fact that the probability from a classifier are the conditional probabilities. Therefore, I do not see the intuition of applying the Brier score to conditional probabilities.


1 Answer 1


The short answer is that it only makes sense to calculate the Brier score for the conditional probabilities, $\hat y = P(y=1|X)$, where $y$ is the outcome, $\hat y$ is your prediction, and $X$ are your predictors. In other words, $\hat y$ is the probability that $y=1$, conditional on this particular value of the predictors, $X$.

The Brier score in this case is just

$$ \frac{1}{N}\sum_i^N (\hat y_i - y_i)^2 $$

What other kinds of probability could there be? The only other option here is the marginal probability, $P(y=1)$. We can estimate this by simply counting the proportion of times $y=1$ in the data. Clearly, it doesn't make sense to use this value when calculating the Brier score!

Would the classifier with smaller Brier score provide a rather better reliability curve?

Yes. If your classifier predicts $\hat y = 1$ in all cases where $y = 1$ and $\hat y = 0$ where $y = 0$, it has a Brier score of $0$. If it does the opposite, it has a score of $\pm1^2 = 1$. In most cases, such perfect predictions won't be possible, but a good classifier can still be well calibrated, for instance by predicting $\hat y = 0.5$ in cases where $y = 1$ half the time and $y=0$ the rest. A classifier that does this will have the lowest possible Brier score on your data.

  • 2
    $\begingroup$ Why doesn’t it make sense to calculate Brier score for a model that always guesses the prior probability? $\endgroup$
    – Dave
    Commented Sep 7, 2020 at 13:04
  • $\begingroup$ I mean, you could do it, and it makes sense mathematically, but it doesn't tell you anything you want to know. My main point is that @ABK was possibly a bit confused about the difference between conditional and marginal probability. $\endgroup$
    – Eoin
    Commented Sep 7, 2020 at 13:48
  • $\begingroup$ To elaborate, if you decided to plug marginal $P(y)$ into the Brier score calculations instead of $P(y|X)$, you would get a score close to 0 if the data is very imbalanced (most zeros, or mostly ones), and a score of $\pm 0.5^2 = 0.25$ if it's 50% zeros and 50% ones. $\endgroup$
    – Eoin
    Commented Sep 7, 2020 at 15:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.