Hello,

I have a few questions regarding interaction terms.

1. If I have my final logistic regression model below, Logistic regression Number of obs = 189
LR chi2(5) = 29.56
Prob > chi2 = 0.0000
Log likelihood = -102.55384 Pseudo R2 = 0.1260

------------------------------------------------------------------------------
low | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
age | .9383918 .0332708 -1.79 0.073 .8753963 1.00592
lwd | 2.54087 .9815007 2.41 0.016 1.191724 5.417377
smoke | 1.585579 .5509269 1.33 0.185 .802469 3.132905
ht | 4.017858 2.57528 2.17 0.030 1.143957 14.1117
ptd | 4.322268 1.93573 3.27 0.001 1.796804 10.39735
_cons | .8722474 .7224864 -0.17 0.869 .1720227 4.422763
------------------------------------------------------------------------------
Note: _cons estimates baseline odds.


2. and now I want to add interaction term between smoke and lwd
  • logistic low age smoke##lwd ht ptd
Logistic regression Number of obs = 189
LR chi2(6) = 32.20
Prob > chi2 = 0.0000
Log likelihood = -101.23846 Pseudo R2 = 0.1372

------------------------------------------------------------------------------
low | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
age | .9433269 .0337749 -1.63 0.103 .8793985 1.011903

smoke |
smoker | 2.263877 .9304785 1.99 0.047 1.011579 5.066477
1.lwd | 4.67193 2.53338 2.84 0.004 1.614089 13.52275
|
smoke#lwd
smoker#1 | .2850451 .2207858 -1.62 0.105 .0624602 1.30084
|
ht | 3.719531 2.426126 2.01 0.044 1.035805 13.35668
ptd | 4.002717 1.800033 3.08 0.002 1.657933 9.663683
_cons | .6733606 .5741697 -0.46 0.643 .1266002 3.581468
------------------------------------------------------------------------------
Note: _cons estimates baseline odds.

3. Can I report from my first final model that lwd (OR 2.54), ht (OR 4.08) and ptd (OR 4.32) are the risk factors for low birth, and from my second model, can I just explain about the interaction between smoke and lwd only (OR 0.28)? or should I use the second model to be my final model and report the results from the second model? if then, how should I interpret smoker 1. lwd and smoke#lwd altogether? because in the first model, smoke wasn't a significant risk factor but in the second model it is significant.

4. how to decide which interaction terms to include in a multivariate logistic regression model? which analysis should I run to choose interaction terms?

Thank you for your time and help in advance.