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Abstract
A sample size calculation for logistic regression involves complicated formulae. This
paper suggests use of sample size formulae for comparing means or for comparing proportions
in order to calculate the required sample size for a simple logistic regression model.
One can then adjust the required sample size for a multiple logistic regression model
by a variance inflation factor. This method requires no assumption of low response
probability in the logistic model as in a previous publication. One can similarly
calculate the sample size for linear regression models. This paper also compares the
accuracy of some existing sample-size software for logistic regression with computer
power simulations. An example illustrates the methods.