REGRESSION QUESTION - need residuals

#1
I have done a series of regressions, all of which have slopes very close to O. I want to perform a Mann-Whitney U tet for independent samples between the residuals of my original regression and one generated from the same data when the slope is equal to O to see if they are significantly diferent. I am using SPSS 11.0 and can generate the residuals for the original regression, but how do get residuals for a regression of the same data with slope = 0 for comparison testing?

Thanks
 

JohnM

TS Contributor
#2
The problem with this approach is that it's pretty much impossible to get statistical software to "generate" a line that doesn't "fit" the data, because there are an infinite number of possibilities.

Additionally, what you're proposing is merely a "roundabout" way of doing what regression analysis tells you anyway....

Some approaches you could take, any of which are fraught with pitfalls (i.e., they're arbitrary):

If you set the slope (b) = 0 for the linear equation y = bx + a, you'll be left with a simple equation such as y = a, where "a" is the y-intercept. Subtract "a" from the y-coordinate of all the data points, and you've got the residuals.

You could also set "a" to be the mean of y, and then do the same subtraction as above.

You could also set "a" to anything you want....

In any case, I think you'll find that the Mann-Whitney test will be significant as well (it's very comparable in power to the t-test when n is large), and quite frankly I'm not sure exactly what this is telling you....