The coefficient for the product (interaction) term becomes an "offset" for the slope coefficient of the group coded as 0. For your case, the zero-coded group slope is, say, .2339, then the offset slope (interaction) coefficient would be -.0164. The t test for this coefficient is exactly the test you want for the difference in slopes.

On Thu, 9 Nov 2000 09:08:12 -0800 Dale Glaser <glaser@pacific-science.com> wrote:

> On the face of it this seems reasonable, however, to put this in context, a > study I was recently involved in, the unstandardized slope for a > fibromyalgia group was .2175 and for arthritic group was .2339, and when > interested in comparing the difference between the slopes for the two > groups, I'm not clear how creating an interaction term with the dummy coded > vector will ultimately answer the research question..........dale > > -----Original Message----- > From: William B. Ware [mailto:wbware@email.unc.edu] > Sent: Thursday, November 09, 2000 8:53 AM > To: Dale Glaser > Cc: SPSSX-L@LISTSERV.UGA.EDU > Subject: Re: comparing b's or beta's > > Would it be appropriate to "dummy" the gender variable and include it > along with its interactions... the tests on the interactions would be > tests for the equality of the regression coefficients, would they not? > > WBW > > __________________________________________________________________________ > William B. Ware, Professor and Chair Educational Psychology, > CB# 3500 Measurement, and Evaluation > University of North Carolina PHONE (919)-962-7848 > Chapel Hill, NC 27599-3500 FAX: (919)-962-1533 > http://www.unc.edu/~wbware/ EMAIL: wbware@unc.edu > __________________________________________________________________________ > > On Thu, 9 Nov 2000, Dale Glaser wrote: > > > Wim.......in the past I have used the formula from Cohen and Cohen (1983) > on > > page 56 when comparing the significance of the difference between > > independent Betas....Zar (1999) in chapter 18 (p. 360) in his > > Biostatistical Analysis text also has a section on comparing two slopes; > > however, and unfortunately I did not save it, I know some one developed a > > macro for the test of two independent betas..............hope this > > helps..........dale glaser > > > > -----Original Message----- > > From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of > Wim > > Beyers > > Sent: Wednesday, November 08, 2000 11:22 PM > > To: SPSSX-L@LISTSERV.UGA.EDU > > Subject: comparing b's or beta's > > > > Dear all, > > > > After having applied a hierarchical regression procedure in two > > groups separately (boys and girls) I want to compare the estimated > > parameters between the two groups, e.g. the b's (regression weights) or > > beta's (standardized). Is there any way to do it, either with computer > > power, either by hand (e.g. a kind of z-test like we have for comparing > > correlations) ? > > Someone with experience in this matter ? > > -- > > Wim Beyers > >

---------------------- C. Mitchell Dayton, Professor Department of Measurement & Statistics and Joint Program in Survey Methodology cd4@umail.umd.edu