interpreting significant interactions in Multiple Regression

Hi everybody,

I'm a master's student who's working on the last analysis for my thesis

project. What I'm wondering is how to interpret significant
interactions in multiple regression. Below is my SPSS output:

Coefficients(a)
Model B Std. E Beta t Sig.
1 (Constant) 2.902 1.120 2.590 .011
BPSMED .145 .093 .183 1.564 .121
NATU .362 .124 .423 2.928 .004
MAST .047 .142 .037 .333 .740
OBFFI -.062 .117 -.048 -.530 .597
BA -.462 .173 -.287 -2.678 .009
BAHPRM -.322 .172 -.214 -1.876 .063
BA*MAST .662 .219 .324 3.024 .003
BA*NATU -.371 .185 -.284 -2.008 .047
a Dependent Variable: TOTSAT


BA is a dichotomous variable (0=MW group, 1=OB group). I know that
because the interaction terms (BA*MAST and BA*NATU) are significant,
that the relationship between MAST and TOTSAT depends on level of BA,
and the relationship between NATU and TOTSAT depends on level of BA. I
have been doing lots of reading about interaction terms in MR (e.g.,
Aiken & West, 1991) and am unsure about how to properly graph the
interaction terms, or if this is even necessary to make an interpretive

statement.


The way I've interpreted them (that I am unsure of) is by substituting
in values of the continuous predictor variable at one SD above and
below the mean in the regression equation (containing only the terms
that make up the interaction and the interaction term) and then
plotting the slopes for each category of the dichotomous variable (MW
or OB). Doing this resulted in the following calculations for the
BA*NATU interaction:


When BA = 1 and NATU is low:
TOTSAT = constant - .46(BA) + .36(NAT) - .37(BA*NAT)
= 2.902 - .46(1) + .36(3.81) - .37(3.81)
= 2.402


When BA = 1 and NATU is high:
TOTSAT = 2.902 - .46(1) + .36(5.59) - .37(5.59)
= 2.382


When BA = 0 and NATU is low:
TOTSAT = 2.902 - .46(0) + .36(3.81) -.37(0)
= 4.27


When BA = 0 and NATU is high:
TOTSAT = 2.902 - .46(0) + .36(5.59) - .37(0)
= 4.91


I graphed these values and interpreted the differences to mean that
while a high score on NAT slightly decreases TOTSAT for the OB group, a

high score on NAT increases TOTSAT for the MW group. Can someone please

tell me if this is correct or if I am completely offbase here???


Another related question I have is how should I interpret the
significant (and non significant) variables that comprise the
interaction terms (BA and NATU are significant, MAST is not)? I read
these are referred to as the main effects, but what do they mean in the

presence of a significant interaction?


Any help would be greatly appreciated!! Thanks.


K

.

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