# Logistic regression sub-group size parameters

Someone in my lab has a sample of 500 older kids and he wants to investigate what factors are related to the probability that they will bully. Groups:

        bullied  not
bully       22    28
don't      200   250


They created an interaction variable using age*not bullied/bullied and added 14 other "predictors" to the mode. Interaction of age and bullying is sig but the prototypical plot looks weird, the group who were bullied have a 0 slope by age and the line starts and ends at 0. My guess is the the bullied/bullying group is way too small for all the info in the model. IF so, what are the group size parameters or ratio to total group parameters for logistic regression? Thank you very much!

• Could you explain what you mean by a 'prototypical plot'? Commented May 2, 2012 at 14:15
• The plot generated to indicate any interaction effect - pls see link - google.com/… Commented May 2, 2012 at 14:36
• I don't really follow the question. Looking at the data reported here, you have a simple 2x2 contingency table setup. I'm not sure how that works w/ whatever was done next. Did they try to turn this into multinomial logistic regression? I don't think that would be appropriate. Also, can you post the plot that you think is problematic? Commented May 3, 2012 at 14:12
• @gung - I think what we have is a bullying/non-bullying outcome with predictors that include whether bullied, age, and 14 others, plus the interaction term. I agree that we'd need to see the plot to comment on it. Commented May 3, 2012 at 14:51
• Something appears to have gone awry as from your data, the probability of bullying among those who were bullied is 22/222 = 0.099, but the plot shows it as 0. Suggests to me that the problem must be related to the 14 other 'predictors'. I suggest you try taking them all out initially, then adding them back one at a time to try to indentify what's causing the problem. Commented May 3, 2012 at 15:11