diff --git a/docs/source/theory/sa.rst b/docs/source/theory/sa.rst
index a92ff5e3..2b726b1d 100644
--- a/docs/source/theory/sa.rst
+++ b/docs/source/theory/sa.rst
@@ -47,7 +47,7 @@ parameter has on the variance of the model:
 
 .. math::
 
-    S_i = \frac{\mathbb{V}[\mathbb{E}[Y | Q_i]}{\mathbb{V}[Y]}.
+    S_i = \frac{\mathbb{V}[\mathbb{E}[Y | Q_i]]}{\mathbb{V}[Y]}.
 
 Here, :math:`\mathbb{E}[{Y | Q_i}]` denotes the expected value of the output :math:`Y` when parameter
 :math:`Q_i` is fixed.
@@ -90,4 +90,4 @@ while for a one dimensional output we get Sobol indices for each point in time.
 To better be able to compare the Sobol indices across such features,
 we therefore calculate the average of both the first order Sobol
 indices :math:`\widehat{S}`,
-and the total order Sobol indices :math:`\widehat{S}_{T}`.
\ No newline at end of file
+and the total order Sobol indices :math:`\widehat{S}_{T}`.