Hello, fellow researchers! I'm hoping to find someone well familiar with Firth's logistic regression. I am trying to analyse whether certain emotions predict behaviour. My outcomes are 'approached', 'withdrew', & 'accepted' - all coded 1/0 & tested individually. However, in some conditions the outcome behaviour is a rare event, leading to extremely low cell frequencies for my 1's, so I decided to use Firth's method instead of standard logistic regression.
However, I can't get the data to converge & get warning messages (see below). I've tried to reduce predictors (from 5 to 2) and increase iterations to 300, but no change. My understanding of logistic regression is superficial so I have felt too uncertain to adjust the step size. I'm also not sure how much I can increase iterations. The warning on NAs introduced by coercion I have ignored (as per advice on the web) as all data looks fine in data view.
My skill-set is only a very 'rusty' python coding, so I can't use other systems. Any SPSS friendly help would be greatly appreciated!
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Warning messages:
1: In dofirth(dep = "Approach_Binom", indep = list("Resent", "Anger"), :
NAs introduced by coercion
2: In options(stringsAsFactors = TRUE) :
'options(stringsAsFactors = TRUE)' is deprecated and will be disabled
3: In (function (formula, data, pl = TRUE, alpha = 0.05, control, plcontrol, :
logistf.fit: Maximum number of iterations for full model exceeded. Try to increase the number of iterations or alter step size by passing 'logistf.control(maxit=..., maxstep=...)' to parameter control
4: In (function (formula, data, pl = TRUE, alpha = 0.05, control, plcontrol, :
logistf.fit: Maximum number of iterations for null model exceeded. Try to increase the number of iterations or alter step size by passing 'logistf.control(maxit=..., maxstep=...)' to parameter control
5: In (function (formula, data, pl = TRUE, alpha = 0.05, control, plcontrol, :
Nonconverged PL confidence limits: maximum number of iterations for variables: (Intercept), Resent, Anger exceeded. Try to increase the number of iterations by passing 'logistpl.control(maxit=...)' to parameter plcontrol
Please inform us of the size of your model as well as the size of your data set. Best wishes, David Booth
In addition you might also take a look at the attached which may be useful if you want to try to reproduce your SPSS results. This might give a clue to your SPSS problem or even solve it for you. Best wishes David Booth
You could try with this program. Or if you can send me the data, I can try for you.
www.lerenisplezant.be/fitting.htm
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