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Re: st: interpretation for negative and positive slope combination of interaction term

From   David Crow <>
Subject   Re: st: interpretation for negative and positive slope combination of interaction term
Date   Thu, 9 May 2013 17:34:34 -0500

Yes, Nahla and David Hoaglin were absolutely right.  Many apologies.


On Thu, May 9, 2013 at 5:06 PM, David Hoaglin <> wrote:
> Dear Nahla,
> You are correct about the slope and intercept, as shown in Figure 7.8
> of that book.  In David Crow's simplified model, B2 does not
> contribute to the slope for MV.
> A major part of your difficulty comes from trying to make ratio
> interpretations of coefficients in a model whose coefficients can tell
> you about differences.  One usual way to get coefficients that have a
> straightforward interpretation as ratios is to analyze the dependent
> variable in a logarithmic scale.  If it is not appropriate to work
> with your dependent variable in a log scale, you may be able to use a
> generalized linear model with a log link function.
> David Hoaglin
> On Thu, May 9, 2013 at 3:51 PM, Nahla Betelmal <> wrote:
>> Thanks for the reply David, but I think there is something not quite
>> right. If you check this file, p.133 figure 7.8
>> You will notice that in order to get the slope for the group with
>> Dummy= 1 (overconfident manager in my case), we should add the
>> coefficient of beta and gama ( MV and OC*MV in my case) , and to get
>> the intercept for those managers we should add the alpha and y ( my
>> model intercept and the coefficient of OC )
>>  I found the same in other files, the problem is that all the examples
>> they provide both beta and gama are positive which makes the addition
>> and interpretation process easy.
>>  my question is how to add and interpret when one is positive and the
>> other is negative . As the effect of MV on realistic managers is -
>> 0.0566  and on  overconfident managers is + 0.0596 , it moves from
>> negative to positive 0.003 (again the interaction is significant
>> although at 10 level). how many times MV effect overconfident managers
>> more than other managers ?
>> I would really appreciate help in that
>> Many thanks
>> Nahla
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