Why are there so many missing values in short (SM variables), long term morbidity (MB variables) activities of daily living (AD variables), and the smoking, chewing tobacco and drinking variables in both surveys?

Questionnaires were designed to reduce respondent burden. Hence at the start of the section, interviewers record the names of the respondents who engage in the activity or have the illness. Those who do not are coded as missing so as to excuse them from further questions. To clarify here is an example concerning illness. When asking about fever, cough and diarrhea, there was an introduction: “We would like to learn about the health of the various family members in this household, including very young children over the last month. We are interested in three specific illnesses: fever, cough and diarrhea. Has anybody been ill with any of these illnesses in the last month?” Then the interviewer asked, “Can you tell me names of all those that had this illness?” This allowed the interviewer to list all members suffering from illness or activity limitations. Hence it can be assumed that those not listed in this section, did not have the illness or limitation. Similarly, when the question is about behavior, those who do not engage in the behavior are coded as missing. PLEASE RECODE THE MISSING VALUES FOR THESE VARIABLES TO 0.