Common classification problems in Machine Learning (ML) are binary and multi-class (Sokolova and Lapalme, 2009). For binary classification, we have accuracy, precision, recall, and a combination of precision and recall which is so-called $F_1$ score. However, the precision and recall from binary classification cannot be utilized literally to measure multi-class classification.

To measure the performance of multi-class classification, two important modifications on precision and recall of binary classification are introduced. Their names are macro-average and micro-average. Therefore, for example, the precision of multi-class classification shall become macro-average precision and micro-average precision.

Let’s begin with an example of multi-class classification with $4$ classes ($0$, $1$, $2$, and $3$). Suppose we have $\text{our predictions}$ and the $\text{true labels}$ for five data instances as follows:

$$\begin{align} \text{our predictions} &= [ 0, 0, 2, 2, 3 ], \\ \text{true labels} &= [ 0, 1, 3, 3, 3 ] \end{align}$$

Our first prediction is $0$ and the true label is $0$. Next, our second prediction is $0$ and the true label is $1$. Our third prediction is $2$ while the true label is $3$ and so on. Let’s denote

$$\begin{align} tp_i &= \text{true positive for class }i \; (i = 0,1,2,3), \\ fp_i &= \text{false positive for class }i \; (i = 0,1,2,3). \end{align}$$

After counting the true and false positives for each class, we obtain

$$\begin{align} tp_0 &= 1, \; tp_1 = 0, \; tp_2 = 0, \; tp_3 = 1, \text{ and} \\ fp_0 &= 1, \; fp_1 = 0, \; fp_2 = 2, \; fp_3 = 0. \end{align}$$

As we’ve already known, $\text{precision}$ for class $i$ ($\text{precision}_i$) is defined as follows:

$$\begin{equation} \text{precision}_i = \frac{tp_i}{tp_i + fp_i}, \text{ for }i = 0,1,2,3. \tag{1}\label{eq:precision-formula} \end{equation}$$

Therefore, employing Equation $\eqref{eq:precision-formula}$, we get

$$\begin{equation} \text{precision}_0 = 0.5, \; \text{precision}_1 = 0, \; \text{precision}_2 = 0, \; \text{precision}_3 = 1. \tag{2}\label{eq:precision-results} \end{equation}$$

As explained in “Micro Average vs Macro average Performance in a Multiclass classification setting”, the macro-average precision ($\text{precision}_M$) for $4$ classes is defined as

$$\begin{align} \text{precision}_M &= \frac{\sum_{i=0}^{3}{\text{precision}_i}}{4} \\ &= \frac{0.5 + 0 + 0 + 1}{4} \\ &= 0.375. \end{align}$$

However, the micro-average precision ($\text{precision}_\mu$) for $4$ classes is defined as

$$\begin{align} \text{precision}_\mu &= \frac{\sum_{i=0}^{3}{tp_i}}{\sum_{i=0}^{3}{(tp_i + fp_i)}} \\ &= \frac{1+0+0+1}{2+0+2+1} \\ &= 0.4. \end{align}$$

If there is a class imbalance problem, one of the options will be using weighted macro-average as performance metrics. The weighted macro-average precision ($\text{precision}_{WM}$) for $4$ classes is defined as

$$\begin{equation} \text{precision}_{WM} = \sum_{i=0}^{3} \text{weight}_i \times \text{precision}_i \tag{3}\label{eq:weighted-precision} \end{equation}$$

with $\text{weight}_i$ is the weight assigned to class $i$ as follows:

$$\begin{equation} \text{weight}_i = \frac{tp_i + fp_i}{\sum_{i=0}^{3}{(tp_i + fp_i)}} \tag{4}\label{eq:weight} \end{equation}$$

Using Equation $\eqref{eq:weighted-precision}$ and $\eqref{eq:weight}$, we compute the weighted macro-average precision as follows:

$$\begin{align} \text{precision}_{WM} &= \frac{2}{5} \times 0.5 + 0 \times 0 + \frac{2}{5} \times 0 + \frac{1}{5} \times 1 \\ &= 0.4. \end{align}$$

Next, we shall show that the weighted macro-average precision does equal to the micro-average precision.

$$\require{cancel} \begin{align} \text{precision}_{WM} &= \sum_{i=0}^{3} \text{weight}_i \times \text{precision}_i \\ &= \sum_{i=0}^{3} \frac{tp_i + fp_i}{\sum_{i=0}^{3}{(tp_i + fp_i)}} \times \frac{tp_i}{tp_i + fp_i} \\ &= \sum_{i=0}^{3} \frac{\cancel{tp_i + fp_i}}{\sum_{i=0}^{3}{(tp_i + fp_i)}} \times \frac{tp_i}{\cancel{tp_i + fp_i}} \\ &= \sum_{i=0}^{3} {\frac{tp_i}{\sum_{i=0}^{3}{(tp_i + fp_i)}}} \\ &= \frac{\sum_{i=0}^{3}{tp_i}}{\sum_{i=0}^{3}{(tp_i + fp_i)}} \\ &= \text{precision}_\mu. \end{align}$$

Finally we have reached the end of this post. In brief, we have shown how to compute macro-average, micro-average, and weighted macro-average. Moreover, we have also shown that the micro-average equals to weighted macro-average.

References

Sokolova, M. and Lapalme, G. (2009). A systematic analysis of performance measures for classification tasks. Information Processing & Management, 45(4):427 - 437.