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'''Random digit dialing''' ('''RDD''') is a method for selecting people for involvement in telephone [[statistical survey]]s by generating [[telephone number]]s at [[random number generation|random]]. Random digit dialing has the advantage that it includes unlisted numbers that would be missed if the numbers were selected from a [[phone book]]. 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]]<ref>{{cite journal|url=http://bbs.vcsnet.com/pdf/RegistrationBasedSampling.pdf|title=Enough Already with Random Digit Dialing: A Proposal to Use Registration-Based Sampling to Improve Pre-Election Polling|author1=Donald P. Green |journal=Vcsnet.com |archivedate=September 28, 2007|archiveurl=https://web.archive.org/web/20070928082303/http://bbs.vcsnet.com/pdf/RegistrationBasedSampling.pdf|author2=Alan S. Gerber |date=2002-05-05}}</ref> and selection of [[experimental control]] groups.<ref>{{cite journal|url=http://aje.oxfordjournals.org/cgi/content/abstract/135/2/210|archive-url=https://web.archive.org/web/20070322020530/http://aje.oxfordjournals.org/cgi/content/abstract/135/2/210|url-status=dead|archive-date=2007-03-22|title=Evaluation of Random Digit Dialing as a Method of Control Selection in Case–Control Studies|author1=Sara H. Olson |author2=Jennifer L. Kelsey| journal=[[American Journal of Epidemiology]]|date=1992-01-15|display-authors=etal}}</ref>
When the desired coverage area matches up closely enough with [[country code]]s and [[area code]]s, random digits can be chosen within the desired area codes. In cases where the desired region doesn't match area codes such as [[electoral district]]s, 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), [[local number portability|number portability]], and [[VoIP]] have begun to decrease the ability for RDD to target specific areas within a country and achieve complete coverage.
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