Let's say I have a 2 variable data set x and y. The linear fit through the line is some y(x). Let's say y=x for simplicity. So a residual is defined as the actual value minus the estimated value (using y). My question is, let's say for one x value there are two y values. e.g., for x=2; y=4 and y=6 If you wanted to calculate the residual at x=2, would you have two separate values or is there someway to combine it? The estimated value would be y=2. residuals are: 2-4 and 2-6, which is -2 and -4. So in other words do you add the two values?