Term Definition
Undocumented code number
A code that is not authorized for a particular question. For instance, if a question that records the sex of the respondent has documented codes of "1" for female and "2" for male and "9" for "missing data," a code of "3" would be an "undocumented code."
Unique Identification Number

A unique number that identifies an element (e.g. serial number). That number sticks to the element through the whole survey lifecycle and is published with the public dataset. It does not contain any information about the respondents or their addresses.


(See Sampling Unit)
Unit nonresponse
An eligible sampling unit that has little or no information because the unit did not participate in the survey.
units (Sampling units)

Elements or clusters of elements considered for selection in some stage of sampling. For a sample with only one stage of selection, the sampling units are the same as the elements. In multi-stage samples (e.g., enumeration areas, then households within selected enumeration areas, and finally adults within selected households), different sampling units exist, while only the last is an element. The term primary sampling units (PSUs) refers to the sampling units chosen in the first stage of selection. The term secondary sampling units (SSUs) refers to sampling units within the PSUs that are chosen in the second stage of selection.

Universe statement

A description of the subgroup of respondents to which the survey item applies (e.g., “Female, ≥ 45, Now Working”).

Unwritten language
An unwritten language is one which does not have a standard written form used by the native speakers of the language.
Usability testing
Evaluation of a computer-assisted survey instrument to assess the effect of design on interviewer or respondent performance. Methods of evaluation include review by usability experts and observation of users working with the computer and survey instrument.