Hello,

just in case my question is trivial, I am a novice at econometric and Stata. I did my research in order to answer this question by myself but I rather get more confused. So I would really appreciate your help!

I run an intreg-model with an interaction term of two continuous variables. The regression output shows that the interaction term is significant at least with p = 0.058.
While the main effect is negative the interaction term has a positive coefficient.
In order to visualize the effects I calculated predictive margins and used the marginsplot. The predicted margins are all significant as well.
According to this graph the moderator weakens the negative main effect and at high levels of the moderator the effect even becomes positive.

Until know I assumend that having a significant interaction term is enough to prove support for my Hypothesis.
However, I read something about a simple slope test to further verify.
My first question is: Is a simple slope test really necessary ?

In order to get the simple slope values I calculated marginal effects.


Code:
  margins, dydx(IV) at (MODERATOR = (-.79112649 .03806308 .86725265 1.6964421 2.5256317 3.3548212))

Average marginal effects                        Number of obs     =        907
Model VCE    : OIM

Expression   : Linear prediction, predict()
dy/dx w.r.t. : MP

1._at        : MODERATOR =   -.7911265

2._at        : MODERATOR =    .0380631

3._at        : MODERATOR  =    .8672526

4._at        : MODERATOR  =    1.696442

5._at        : MODERATOR  =    2.525632

6._at        : MODERATOR  =    3.354821

------------------------------------------------------------------------------
             |            Delta-method
             |      dy/dx   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
IV         |
         _at |
          1  |  -.3028635   .1648471    -1.84   0.066    -.6259579    .0202309
          2  |  -.1928102   .1186903    -1.62   0.104    -.4254389    .0398185
          3  |  -.0827569   .0879611    -0.94   0.347    -.2551574    .0896436
          4  |   .0272964   .0901326     0.30   0.762    -.1493602    .2039529
          5  |   .1373496    .123481     1.11   0.266    -.1046687     .379368
          6  |   .2474029   .1706112     1.45   0.147     -.086989    .5817948
------------------------------------------------------------------------------

.
I standardized the variables. Thus -.79112649 actually represents the value zero for the moderator.
So according to these results there is only a significant marginal effect if the moderator is zero.

What does this mean for my interpretation of the interaction effect and for the explanatory power of my figure showing the predictive margins?


I am very grateful for any help!
I hope I have provided all necessary information.

Many thanks, Sara.