Minor changes

.gitignore

@@ -1,2 +1,3 @@
 env/
-build/
+build/
+*\#*

source/statistics/introduction.rst

@@ -1,4 +1,13 @@
 Introduction
 ==================
 
-dd
+Metrics
+----------------
+
+* **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.
+
+Variance
+^^^^^^^^^^^^^^
+
+Variance can be seen as the expected squared deviation from the mean of a random variable :math:`X`.