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I am trying to interpret my interaction effects, which are all negative. One example: Experience (variable A) x absolute size of the acquired knowledge base (variable B): B= -0.002, exp(B)= 0.998.

Can I interpret this interaction in the following way: As variable B decreases, the effect of A increases. How can this interaction be interpreted in terms of exp(B)?

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When variable A increases by one unit the effect of variable B decreases by a factor .998 or $(.998-1)\times 100\% = -.2\%$.

There are many examples of this type of interaction effects, including one for a negative binomial model, here: http://www.maartenbuis.nl/publications/interactions.html

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  • $\begingroup$ I tried to get my mind around this but I still have some questions :). I test the main effect of several IV. However, I also suspect that one of my IV (experience) has a moderates the relationship between the remaining IV and the DV. I have the following main effects for my IVs without interactions in the model: Experience=0.011*, IV B=0.016, IV C=0.106**, IV D=-2.693**, IV E-0.139. In a second model I now added the interactions between experience and the other IVs, leading to the following results: Experience=0.045**, IV A=0.092, IV B=0.146**, IV C=-2.506**, IV D=-0.156 $\endgroup$ – Schwadi Jun 16 '13 at 13:20
  • $\begingroup$ My interactions have to following coefficients: Experience x B=-0.009, Experience x C= -0.002*, Experience x D=-0.020 Experience x E=0.003. Do I understand it right that the coefficients of the main effects are now for when experience has a value of 0? The coefficient of the interaction should then show in which direction the coefficient of the main effect changes. Thus, a negative interaction means that a positive main effect will have an increased coefficient, which shows that there is moderation. A significant interaction coefficient then means that this moderation is significant? $\endgroup$ – Schwadi Jun 16 '13 at 13:26
  • $\begingroup$ In principle yes, except that I would not say something like "this moderation is significant". This often only distracts from the real message of your paper. I would stay a lot closer to the real meaning of the variables, something like "if someone has 0 years experience a year increase in education leads to an (exp(b)-1)*100% in/decrease in Y, and for every year increase in experience this effect of education decreases by (exp(a)-1)*100%." $\endgroup$ – Maarten Buis Jun 17 '13 at 7:30

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