diff --git a/education/statistics/Correlation and Regression.md b/education/statistics/Correlation and Regression.md
index 3fbe593..0f14f06 100644
--- a/education/statistics/Correlation and Regression.md	
+++ b/education/statistics/Correlation and Regression.md	
@@ -106,7 +106,9 @@ $$ \hat{y} = \frac{x-\bar{x}}{\sigma_x} * r * \sigma_y + \bar{y} $$
 The distance of an individual point from the regression line. 
 - If a point is below the line, the error is negative.
 - If a point is above the line, the error is positive.
-- This value is calculated by subtracting the y value of the regression line from the y value of the point. 
+- `residual = observed - predicted` for a given $x$ value 
+- The r.m.s error is the r.m.s size of the errors
+$$ \sqrt{1-r^2}(\sigma_y) $$
 
 
 ---