You are given a logistic regression equation that predicts the probability of having a disease dependent on whether or not three risk factors are present. For any given patient whose chance of disease you want to predict, you are able to find out whether or not they have the first two risk factors, but you have no information either way about the third. In a situation like this, is there any way the regression equation could reasonably be used?
Of course, you can't simply set the third term equal to zero, since that supposes the patient does not have the risk factor. Is there any other option? Say you know that the prevalence of the third risk factor in the patient's population is 25%. Could you set your third independent variable equal to 0.25 and then use the equation to estimate the patient's probability of disease?