I am doing a logistic regression analysis using the glm command in R. It is to identify causes of valve narrowing beyond a certain threshold; 0=no narrowing, 1=narrowed. One of my variables is the size of a medical device that is implanted (range 25-36mm). Sometimes the device isn't implanted and I've left this as a blank field, but of course this is interpreted as a missing field. Not implanting the device seems to have a significant effect using Chi-sq analysis, and the size of the device has a significant effect using a t-test. How do I get around this in a linear regression model?
To make it more complicated I actually have two different makes of the device: "C" and "D" with sizes 25-36mm, another device without a size "S" and then no device "N". Can it all be entered together or is it best to analyze separately outside of regression?
What effect does the "missingness" have on various other variables that are in the analysis?