# Sample size for logistic regression. How to determine the number need for positive and negative cases? [duplicate]

I would like to apply logistic regression for my research. And before that, I want to calculate the minimum number of sample size, positive cases, and negative cases.

http://www.medcalc.org/manual/logistic_regression.php

I came across the above website, stating the below calculation.

"Sample size calculation for logistic regression is a complex problem, but based on the work of Peduzzi et al. (1996) the following guideline for a minimum number of cases to include in your study can be suggested. Let p be the smallest of the proportions of negative or positive cases in the population and k the number of covariates (the number of independent variables), then the minimum number of cases to include is:

N = 10 k / p For example: you have 3 covariates to include in the model and the proportion of positive cases in the population is 0.20 (20%). The minimum number of cases required is

N = 10 x 3 / 0.20 = 150

If the resulting number is less than 100 you should increase it to 100 as suggested by Long (1997)."

So, since my positive cases in the population is 7.4%=0.074; and I have 10 independent variables. Therefore the sample size needed N = 10 * 10 / 0.074 = 1352.

But how can I determine how many positive and negative cases do I need? Or does it mean I would need 1352*50% for positive and negative cases respectively?

What if my available sample include only 500 positive and 7500 negative cases? Would it be inappropriate to run logistic regression?

Thank you !

• Try Googling "rare event logistic regression." Gary King may have done articles that will be helpful. Jan 30, 2015 at 3:06
• Jan 31, 2015 at 3:27
• Another dup: stats.stackexchange.com/questions/162294/… Mar 11, 2019 at 23:06