Random digit dialing (RDD) is a method for selecting people for involvement in telephone statistical surveys by generating telephone numbers at random.
In populations where there is a high telephone-ownership rate, it can be a cost efficient way to get complete coverage of a geographic area.
RDD is widely used for statistical surveys, including election opinion polling[1] and selection of experimental control groups.
In cases where the desired region doesn't match area codes (for instance, electoral districts), surveys must rely on telephone databases, and must rely on self-reported address information for unlisted numbers.
Increasing use of mobile phones (although there are currently techniques which allow infusion of wireless phones into the RDD sampling frame), number portability, and VoIP have begun to decrease the ability for RDD to target specific areas within a country and achieve complete coverage.