Listed Number

A listed telephone number is one that can be retrieved from a telephone company's directory assistance service, and it also may be published in a local telephone directory. The majority of U.S. residential landline (wired) telephone numbers are listed, but a growing proportion are unlisted—more than 40% as of 2008. There is no equivalent concept of “listing” that applies to cell phone (wireless) numbers in the United States, as there are no directory assistance services or telephone directories that are publicly accessible that contain cell phone numbers.

Whether or not a landline telephone number is listed is predictive of the likelihood that a completed interview will be attained with that household in a telephone survey. A greater proportion of interviews are completed with numbers that are listed than are completed with unlisted numbers. There are two primary reasons for this. First, those people who list their telephone number, in general, are more positively disposed to cooperate when they are reached for a telephone survey, compared to those with unlisted numbers. Concern about privacy is one of the factors that explain this. The demographic correlates of whether or not someone lists their number also are related to this, as minorities (who generally have lower cooperation rates in surveys) are less likely than are whites to have a listed landline number.

Second, almost all listed numbers also have an address associated with them. As such, researchers can send advance mailings to these households when they are sampled for a telephone survey to alert them (“warm them up”) to the fact that an interviewer will be calling them. Advance letters with as small an incentive as $2 have been found to raise cooperation rates by approximately 10 percentage points in general population telephone surveys in the United States.

On average, listed telephone numbers require fewer callbacks than unlisted numbers for them to reach a proper final disposition. Thus, the calling rules used by a survey center to process listed numbers should differ from the rules used to process unlisted numbers. However, unless a survey center has their telephone samples screened for listed versus unlisted status or received this information for each number in their sample from their sample vendor, it will not be possible for them to take the listed versus unlisted status into account as their computer-assisted telephone interviewing system processes the callback attempts.