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FIGURE 18-8: Visualizing the complete separation (or perfect predictor) problem in logistic regression.

Figuring Out the Sample Size You Need for

Logistic Regression

Estimating the required sample size for a logistic regression can be a pain, even for a simple one-

predictor model. You will have no problem specifying desired power and α level (see Chapter 3 for

more about these items). And, you can state the effect size of importance as an OR.

Assuming a one-predictor model, the required sample size for logistic regression also

depends on the relative frequency of yes and no outcomes, and how the predictor variable is

distributed. And with multiple predictors in the model, determining sample size is even more

complicated. So for a rigorous sample-size calculation for a study that will use a logistic

regression model with multiple predictors, you may have no choice but to seek the help of a

professional statistician.