Minor changes

source/statistics/metrics.rst

@@ -1,8 +1,16 @@
 Metrics
 ==================
 
-* **Expected value/Espérance**: Noted :math:`\mathbb{E}[X]`, is a **theorical value**. For example, when playing coin
+Expected value
-  flipping, the expected value for getting heads or tails is 0.5.
+---------------
+
+The expected value (*espérance*) noted :math:`\mathbb{E}[X]`, is a **theorical value**.
+For example, when playing coin flipping, the expected value for getting heads or tails is 0.5.
+For a random variable :math:`X` with :math:`n` possible outcomes, with the respective probabilities :math:`p_1,\cdots,p_n` of
+occurring we have:
+
+.. math::
+   \mathbb{E}[X]=x_1p_1+x_2p_2+\cdots+x_np_n
 
 Variance
 ------------------