Understanding Society User Support

Hi. I have a question about the derived housing costs variable in BHPS Wave R:

The derived variable rxphsg is described as giving monthly mortgage or rental costs, net of housing benefit. It is noted that it should only be 0 for houses i)owned outright, ii)lived in rent free, or iii)completely covered by housing benefit. However, 40% of the Wave R sample have a value of 0 for this variable. This strikes me as a little high, even for Wave R (which I understand is a little older than the general UK population) – and even for those under 60, a quarter are apparently living with 0 housing costs. Can you confirm that this variable is correct?

Best,
Rob