1. ## interpreting formulas properly

All my life i seem to interpret formulas incorrect. ive never really tried in school until now. ive been in college for awhile. i know im pathedic. Anyways, i have a relatively simple question. Forgive me because i dont know how to superscribe and XXXX i cant think of the terms. anyways. im donig hte least squares regression line.
E=sum in my equation.

S_x_y=E X_i Y_i

why do i multiply X*Y individually and then sum that entire row?

Why dont i multiply the sum of the X's times the sum of the Y's?

If anyone knows somewhere that i could work on interpretting formulas id appreciate it because it seems that it can be fixed rather quickly while it causes lots of grief.

___________

Also, why sometimes does it show Yhat sub i = bsub0 +bsub1*Xsubi

whats the difference between sub i and sub 1? thanks

2. Originally Posted by brett701
All my life i seem to interpret formulas incorrect. ive never really tried in school until now. ive been in college for awhile. i know im pathedic. Anyways, i have a relatively simple question. Forgive me because i dont know how to superscribe and XXXX i cant think of the terms. anyways. im donig hte least squares regression line.
E=sum in my equation.

S_x_y=E X_i Y_i

why do i multiply X*Y individually and then sum that entire row?

Why dont i multiply the sum of the X's times the sum of the Y's?
Because that is what the notation means, summing the product is different form multiplying the sums and:

$\displaystyle \sum_{i=1}^n x_iy_i$

is defined to mean:

$x_1y_1+x_2y_2+... + x_ny_n$

CB