Data on the sex of each reporting unit (RU) member was determined during the NHIS interview, and was then verified, and, if necessary, corrected during each MEPS interview.

If the respondent was a new RU member or their sex was not ascertained in the NHIS interview, MEPS initially used the first name of the respondent to assign their sex. If the first name gave no clear indication of sex, the reported family relationships were used to assign sex. If the sex of the respondent was still unclear, sex was randomly assigned.

The NHIS method of ascertaining the sex of the respondent, which primarily informs the MEPS version of SEX, is similar to the MEPS method. First, sex of the respondent was inferred from the individual's first name or family relationships. If the sex of the respondent was unclear, the interviewer was instructed to explicitly ask the person's sex. Beginning in 1998, interviewers were told to "enter your best guess" when the respondent either did not know or refused to answer the direct question that was asked about the person's sex.