Re: logistic regression: number of cases per category

Posted by Bruce Weaver on Dec 23, 2010; 12:10pm
URL: http://spssx-discussion.165.s1.nabble.com/logistic-regression-number-of-cases-per-category-tp3316148p3316269.html

student09 wrote

Hello everybody,

I would be glad for advice concerning the followin problem:

The sample size for my logistic regression with four predictor variables appears somewhat small (N= 160) to me. Regarding the dichotomous dependent variable, there are 20 cases for category (a) and 140 cases for category (b). I wonder whether this might induce any problems for the results - is it ok to conduct a logistic regression with just 20 cases in one of the two categories for the dependent variable? Are there any references available regarding adeqaute sample sizes in logistic regression?  

Many thanks!
Jan

Frank Harrell, author of the book "Regression Modeling Strategies", advocates a 20:1 rule, meaning 20 events per candidate predictor variable.  See the section on overfitting here:

   http://biostat.mc.vanderbilt.edu/wiki/Main/ManuscriptChecklist

Personally, I am comfortable relaxing that to 15:1, or even 10:1 at times, although 10:1 is really pushing it.  For more on overfitting, see the nice article by Mike Babyak, available here:

   http://www.class.uidaho.edu/psy586/Course%20Readings/Babyak_04.pdf

HTH.