IHIS Sawment variable NIU

I am using 2012-16 IHIS data to look at adult mental health disparities. I am using the “sawment” variable, but over half of the adult (18-64) subjects are coded as “NIU.” I’m not seeing anything in the documentation that would explain this. Is it maybe because respondents can’t answer for other household adults? Thanks for any guidance you can provide.

The universe for SAWMENT is sample adults age 18+ and sample children age 2 to 17. The variable ASTATFLG identifies the record of a sample adult. A sample adult is the one adult per family who was selected at random by the computerized survey instrument to answer additional health-related questions, under the survey design instituted in 1997. If you narrow your results for SAWMENT to those who were selected as the sample adult, you will find that those who were selected as the sample adult but have no record (code 2 from ASTATFLG) will be NIU.