[statistics] added equation label to correlation coefficient

statistics/lecture/statistics.tex

@@ -455,10 +455,13 @@ data values (e.g. size and weight of elephants) we want to analyze
 dependencies between the variables.
 
 The \enterm{correlation coefficient}
-\[ r_{x,y} = \frac{Cov(x,y)}{\sigma_x \sigma_y} = \frac{\langle
+\begin{equation}
+  \label{correlationcoefficient}
+  r_{x,y} = \frac{Cov(x,y)}{\sigma_x \sigma_y} = \frac{\langle
   (x-\langle x \rangle)(y-\langle y \rangle) \rangle}{\sqrt{\langle
     (x-\langle x \rangle)^2} \rangle \sqrt{\langle (y-\langle y
-    \rangle)^2} \rangle} \] 
+    \rangle)^2} \rangle}
+\end{equation}
 quantifies linear relationships between two variables
 \matlabfun{corr()}.  The correlation coefficient is the
 \determ{covariance} normalized by the standard deviations of the