# Lines of regression

#### Hans Rudel

##### New Member
Im little confused with the following.

there are 12 patients, each is assigned assigned an index I when first administered depending on the severity of their symptoms. Each then administered a drug and 30 days later they are given another index F which will hopefully be lower than I if they are improving.

Calc the equation of the regression line of F on I.

if i take I = x and F = y then i can calc the equation using y = a + bx where b = Sxy/Sxx i get the right answer.

if i switch them ie I = y and F = x, with x = C + Dy where D = Sxy/Syy then i get the wrong answer.

How am i suppose to know which should be x and which should be y?

Thanks very much for your help.

#### Dason

It's true that you get a slightly different regression line if you switch the predictor/response. However you should know ahead of time which is the predictor and which is the response. If you don't then regression probably isn't what you want to do. Correlation might be more appropriate and that doesn't depend on labeling one variable predictor and one as response.

#### Hans Rudel

##### New Member
Thanks very much for taking the time to reply to my post!

The book im reading states that you can use regression even when both the x and y (I and F in the above example) variables are not controlled, which is still a bit confusing in light of what you have said "you should know ahead of time which is the predictor and which is the response".

I guess I is the predictor in my example? (i had assumed both were not controlled).

Thank you for the heads up regarding the use of correlation as an alternative.