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.