Fitting the regression line In Fitting the regression line shouldn t the expression immediately before the one containing Cov Var have 1 n as the multiplier before the x y and before the x 2 terms This is the form of the expression which is often used in computing to generate a straight line fit to set of bumpy data As expressed here it does not work and moreover does not follow mathematically from the preceding expression However with the 1 n terms in place it appears to produce the correct result 1 n is unique to these two terms ONLY and therefore does NOT cancel But then I am a Physicist and not a Mathematician so I may have missed something Chris B 6 45pm PST on 24th Mar 2013 Preceding unsigned comment added by Have a look at the section below By using calculus which steps through a derivation and also mentions multiplying top and bottom by N Incidentally I also studied Physics beta hat I was confused by the formula for I wonder if the second one should have more parentheses around sums like as product has precedence over summation but I am no mathematician nor English so perhaps do not know conventions Can someone look at it and possibly fix it I know it is logical to at least assume the parentheses but this is introductory and should be as precise as possible