If you're only comparing two slopes, can't you just look at the interaction term of an ANCOVA? If it's significant, the slopes are different.
Continuous response = continuous + categorical + contin*categ If your categorical variable has more than two levels (slopes) you're faced with the same question as in ANOVA: which levels are different from which other levels? This is answered with multiple comparisons, in this case, MCs of slopes (Zar). -R Jane Foster wrote: >I realize I'm a little late to this discussion, but I haven't heard anyone >mention the "Extra Sums of Squares" or "Additional Sums of Squares" >principal which can be used to compare slopes and/or intercepts of >different regression models. I don't have a good reference for the >procedure used, and it can require some care in the way the data is set up >to test different hypothesis about how models differ, but I know it is >another possible approach to this problem. > >Jane F. > > > > > >>Your approach is valid ONLY IF you are willing to ignore the fact that the >>slope to which you are comparing your slope is itself an estimate. That >>is >>- you can use your CI to compare to a particular hypothesized value - >>basically testing the hypothesis Ho: beta = beta_0, where beta_0 is some >>hypothesized value, possibly from the literature. However, if you really >>want to see if two slopes are equal, say Ho: beta_1 = beta_2, you are >>better >>off using the test on p. 360 of Zar. This essentially looks at the CI of >>the difference in slopes (b_1 - b_2) to see if it includes 0. >> >>On 8/16/06, David Whitacre <[EMAIL PROTECTED]> wrote: >> >> >>>While we're on regression--I know this is a really dumb question and I >>>should know the answer. But here goes, my ignorance on display: >>> >>>In comparing some regressions to published ones, how do I test for >>>significant difference in slope? I have calculated the 95% C.I. of my >>>slope by using the t distribution applied to the SE of the slope, as >>>described on p. 331 of Zar (1996, 3rd edition). >>> >>>If somebody else's slope is outside of this C.I., are the two slopes >>>significantly different at p = 0.05? That is, I don't have to consider >>>the >>>C.I. on their slope? >>> >>>Thanks much for any enlightenment on this very basic issue. >>> >>>Dave W. >>> >>> >>> >> >> >> > > >
