Why are there sometimes no numbers or coefficients for some of the variables in a regression model? Why does R show NA instead of a coefficient? Normally, this happens when there are categorical variables that are perfectly correlated.
Replace a missing value category with NA in R/RStudio
In this video, we’re going to replace a missing value category like 77, -99, or 999 with NA so that R will ignore it in the analysis. Recoding values as NA tells R that it’s a missing value.