+ Reply to Thread
Results 1 to 7 of 7

Thread: Testing Simple Effects for Generalized Linear Regression

  1. #1
    Points: 41, Level: 1
    Level completed: 82%, Points required for next Level: 9

    Posts
    5
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Testing Simple Effects for Generalized Linear Regression




    Hello,

    I have found significant interaction effects when analyzing a negative binomial generalized linear regression model. However, I am not sure how to further explore the interaction. I am currently using SPSS. How can I conduct follow-up simple effects to examine the interactions? Can I simply add something on to the syntax?

    Thank you!

  2. #2
    Omega Contributor
    Points: 38,289, Level: 100
    Level completed: 0%, Points required for next Level: 0
    hlsmith's Avatar
    Location
    Not Ames, IA
    Posts
    6,992
    Thanks
    397
    Thanked 1,185 Times in 1,146 Posts

    Re: Testing Simple Effects for Generalized Linear Regression

    How are your two variables formatted? I am guessing they are both binary?


    I have not used negbin before, but I am guessing you can look at the odds ratios for one term stratified by the other term. I think it is usual to stratify on the term that may be easier to intervene on. So if I had employment status and alcohol consumption (y/n), I would look at the odds ratio of employ status on outcome for non-drinkers, then do it again but for the drinkers.
    Stop cowardice, ban guns!

  3. #3
    Points: 41, Level: 1
    Level completed: 82%, Points required for next Level: 9

    Posts
    5
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: Testing Simple Effects for Generalized Linear Regression

    I have one continuous variable (risky environment) and one binary variable (gene; carriers vs. non-carriers) predicting alcohol use (continuous). What are you saying makes sense, in my case I should look at the odds ratio of the risky environment among carriers and then again among non carriers. What I am unclear about is how exactly to do this. Should I run another regression (including all covariates?) without the interaction term among only carriers and then again with only non-carriers? Any help would be well appreciated!

  4. #4
    Omega Contributor
    Points: 38,289, Level: 100
    Level completed: 0%, Points required for next Level: 0
    hlsmith's Avatar
    Location
    Not Ames, IA
    Posts
    6,992
    Thanks
    397
    Thanked 1,185 Times in 1,146 Posts

    Re: Testing Simple Effects for Generalized Linear Regression

    So your outcome is continuous and you have a binary*continuous interaction term, correct? If so you wouldn't use odds ratios, but plot the slopes for gene carriers and non-gene carriers on a graph, so you would have two incongruent slopes on the same graph. Since there is interaction you would assume the lines would be disordinal (cross each other). If needed you could slap confidence bands on them as well. See Figure 1 in the below article. That image is very crude, but conveys a vague depiction of an interaction.




    Smith HL, Sidwell RA. Trauma patients over-triaged to helicopter transport in an established rural state trauma system. J Rural Health. 2013;29(2):132-139. (PMID: 23551643)

    Stop cowardice, ban guns!

  5. #5
    Points: 41, Level: 1
    Level completed: 82%, Points required for next Level: 9

    Posts
    5
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: Testing Simple Effects for Generalized Linear Regression

    Exactly, a continuous outcome with a binary*continuous interaction term. My advisor wants me to report (1) the effects of one predictor (that is, an environmental factor) on the DV when another predictor is set to be zero (that is, non-carriers) and (2) the effects when another predictor is set to be one (that is, carriers).

    I see how creating the figure would be a visual aid, but is there any supplemental follow-up tests that will produce a p-value to help support my interaction finding?

    Thank you!

  6. #6
    Omega Contributor
    Points: 38,289, Level: 100
    Level completed: 0%, Points required for next Level: 0
    hlsmith's Avatar
    Location
    Not Ames, IA
    Posts
    6,992
    Thanks
    397
    Thanked 1,185 Times in 1,146 Posts

    Re: Testing Simple Effects for Generalized Linear Regression

    I am guessing you are running a multiple linear regression model, correct? If so, your model would be: Y = Bo + B1(X1) + B2(X2) + B3(X1*X2) + error term.


    Where:
    Y= continuous dependent variable
    Bo = intercept (reference binary group slope)
    B1 = slope for continuous variable
    B2 = slope for binary non-reference group
    B3 = slope for multiplicative interaction


    If you model it this way, if the interaction term is significant then an interaction between the binary and continuous variable is present.
    Stop cowardice, ban guns!

  7. #7
    Points: 41, Level: 1
    Level completed: 82%, Points required for next Level: 9

    Posts
    5
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Re: Testing Simple Effects for Generalized Linear Regression


    Yes, a multiple linear regression. Should I simply calculate that by hand or there a way to do it in SPSS to obtain p-values?

    Thank you!

+ Reply to Thread

           




Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts






Advertise on Talk Stats