Despite the potential value of commercial voter files, they are hardly perfect. The imperfections stem from a variety of sources. At their core, the files are a compilation of official records from each state and the District of Columbia, with the addition of information about both registered and unregistered adults from other sources. But the administration of elections in the U.S. is remarkably decentralized, and the means by which official records are maintained and updated varies, although less so now than in the past. Moreover, the rules and norms governing access to voter records vary considerably from place to place.

Beyond the record of a voter’s registration and turnout, the quality of additional information such as demographic characteristics or a phone number is not uniform and is sometimes unavailable.

One important source of error in voter files is that Americans remain a fairly mobile population and there is no official system to notify elections officials that a voter has moved. (The National Change of Address dataset is maintained by the U.S. Postal Service but is not automatically integrated with election systems in the states.) The companies that compile and market voter data attempt to link voting records of individuals when they move, but the process is complex and far from foolproof. The kinds of people who are most likely to be missed in the voter files when they move do not constitute a random subset of the population, but instead are more likely to be younger, less educated, poorer and nonwhite. Similarly, very mobile Americans are more likely to appear on files in more than one location.

A related source of bias is the fact that voter files systematically miss those who are not registered to vote. Most of the commercial vendors of voting data attempt to include all Americans – registered and unregistered – because many users of voter files are interested in reaching and mobilizing all voting-eligible citizens. But since the files are built initially on official registration records, many of the unregistered fall through the cracks. The unregistered in the U.S. are less likely to have clear digital footprints, due in part to their greater mobility.

Efforts have been made to deal with errors in voter registration records resulting from mobility and other factors. For example, the Electronic Registration Information Center, also known as ERIC, is a nonprofit organization funded by 20 participating state governments to actively align official voter files across state lines to reduce these kinds of errors and to increase access to voter registration for all eligible citizens. (Disclosure: ERIC was formed in 2012 with assistance from The Pew Charitable Trusts, the parent organization of Pew Research Center.)

Some political scientists have also argued that the use of commercial voter files raises important normative questions. Those who believe that the political process benefits from higher levels of citizen participation may see voter files as providing a means for facilitating participation in the political process. While it can be demonstrated that voter files can be instrumental in promoting greater turnout among targeted groups and individuals, it is difficult to know whether their use results in an overall increase in political engagement. Similarly, while greater aggregate participation may be a desirable goal for a democracy, there is evidence in the political science literature that voter files increase inequality in participation because they are used primarily to further mobilize people who are already engaged. If efficiency in the use of campaign resources is a principal goal of practitioners (rather than engaging new or irregular voters), voter files could produce greater inequality in participation by making it easier for campaigns to avoid “wasting” effort on younger or poorer voters who may have a low propensity to participate in the first place.

Beyond the impact that voter files may have on the democratic process, the widespread availability of such detailed information about individuals raises concerns about personal privacy. Pew Research Center studies have found that Americans hold strong views about privacy in everyday life. They worry about the amount of their personal information that is being collected but at the same time are open to providing information in exchange for certain kinds of benefits. Nevertheless, they have little confidence that personal data about them held by businesses and government is secure (see the Methodology section of this report for details about how the Center handled survey respondents’ personal information).

The core data in voter files are the publicly available voting records of individuals. Members of the public may be unaware that voting records are public, but campaigns have long had access to them. What has changed is that they are much more accessible in the digital age due to changes in both government policies and the routine practices of the agencies that administer elections. It is simply more efficient for governments to digitize the records necessary for the orderly administration of registration and voting.

Another change is that it is now much easier to merge voter records with other kinds of digital data, such as that collected by marketing and credit data companies. And it is possible to merge the voter file data, including the financial and marketing data, with data from social media platforms. Together, this information can provide a relatively comprehensive portrait of many individual citizens for use by campaigns and interest groups. Of course, this is just the political equivalent of what marketers are doing to identify and target consumers for specific products and services. But it brings the political process into the ongoing debate about personal privacy, where people often have strong negative reactions to finding themselves the focus of tailored ad campaigns and the like.