diff --git a/education/statistics/Correlation and Regression.md b/education/statistics/Correlation and Regression.md
index a73e5f7..462b59d 100644
--- a/education/statistics/Correlation and Regression.md	
+++ b/education/statistics/Correlation and Regression.md	
@@ -28,7 +28,11 @@ Correlation is between `-1` and `1`. Correlation near 1 means tight clustering,
 ## Calculating $r$ by hand
 Put the $x$ values into $L1$, put the $y$ values into $L2$. 
 
-Convert the $x$ values to standard units. Convert the $y$ values to standard units. Multiply the standard units for each x y pair. Find the average of the values from step 3, this is $r$.
+1. Convert the $x$ values to standard units ($z$). Convert the $y$ values to standard units.
+2. Multiply the standard units for each ($x$, $y$) pair. 
+3. Find the average of the values from step 3, this is $r$.
+$$ z_x = \frac{L_1-\bar{x}}{\sigma_x} $$
+https://www.thoughtco.com/how-to-calculate-the-correlation-coefficient-3126228
 
 # Terminology
 | Term | Definition |