I'm quite familiar with MCAR, MAR and MNAR (NMAR) data but I have just come across a new (for me) term: Structurally Missing Data (SMD).
According to this page, Structurally missing data is data that is missing for a logical reason. They give an example where they ask people if they have kids and then they ask what the age of the youngest kid is. Some people answer "no" to the first question and then leave the second one blank (as there is no kids, so no age).
But here there is a relationship between having kids and the age of youngest. Since there is a relationship, this makes the data MAR.
So, my question is, is SMD a subset of MAR, or is there a reason why it is not?