Yep - if you have the data that generated the two slopes (the original question referred to getting slopes from the literature), this tests for equal slopes.
On 8/23/06, Ron E. VanNimwegen <[EMAIL PROTECTED]> wrote: > > 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. > >>> > >>> > >>> > >> > >> > >> > > > > > > >
