Missing data in sample surveys are of two general forms. Unit nonresponse occurs when no information is available to the survey for an entire sample unit, such as a person, or household, or hospital. Some information may, however, be available from other kinds of records such as those used to define the sample frame. The reasons for unit nonresponse vary; for example, a person may refuse to respond, be away from home, or be impossible to locate. Typically, this form of nonresponse is handled in part by a call-back strategy. That is, the interviewer makes repeated attempts to contact the unit. If the call-back strategy fails, or is not feasible, weights can be assigned to the responding units (Cochran, 1977).