A newly released Government Accountability Office review of Census Bureau follow-up efforts to reduce errors in the 2010 Census raises an issue familiar to survey researchers: The problem of reaching the growing share of Americans who have only cell phones and not landlines.

Although the 2010 Census was mainly a mailback operation, the Census Bureau also telephoned nearly 7.4 million households to seek clarification of confusing or potentially incorrect information on their census forms.This operation, called Coverage Follow-up (CFU), was completed on schedule and under budget, GAO concluded in a report that was one of three evaluations released by the agency the week before the Census Bureau is scheduled to publish the first counts from the 2010 Census.

The GAO evaluation also found the bureau completed another follow-up task, Field Verification, on time and under budget. But the agency’s evaluations raised questions about some aspects of the design of both operations, which had as their goal a reduction in the number of Americans who were missed, counted more than once or counted in the wrong place by the 2010 Census.

The problems encountered with telephone follow-up were among a number of issues discussed in the GAO report, but they are the focus of this posting because they are relevant to any research that relies on reaching respondents by telephone. As GAO points out, the National Center for Health Statistics has found that more than 24% of U.S. households use only wireless telephones, not landlines, and work by the Pew Research Center also has found that when survey-takers call only landlines, it can skew their results. Including cell phones in a survey sample adds significantly to the cost and complexity of conducting research.

In the Coverage Follow-up operation, the Census Bureau contacted households with census form problems by telephone, which enabled it to reach more households than it would have been able to with more expensive in-person visits. It also used a commercial database to identify landline phone numbers that were missing or not valid, so as to target its calls more effectively. The Census Bureau only used landlines from the commercial database “due to concerns about not knowing the geographic location of wireless phones it might dial and the possible financial burden on wireless customers of unsolicited calls,” the GAO report said.

In general, GAO said the bureau’s strategy “should contribute to a greater number of coverage errors being removed from official census tabulations compared to 2000.” But the GAO said those tactics still may not reduce what is called the “differential undercount”– that is, the tendency for a greater share of people from some demographic groups to be missed by the census. That is because households that use only cell phones are likely to be the types of households that census-takers have trouble reaching in the first place. Cell-only households are more likely than the population overall to be young, black or Hispanic, renters and/or low-income.

The GAO urged the Census Bureau to expand its efforts to track and assess the implications of trends such as the increasing share of cell-only households, so as to be better able to deal with them. For example, “the Bureau might need to adopt strategies for increasing the number of usable telephone numbers provided by census respondents or revisit its specific rules concerning when to dial numbers.”

According to Census Bureau comments included in the GAO report, bureau officials say they recognize the need to pay more attention in the future to trends away from landlines and toward wireless-only coverage…. For the 2020 Census research and testing program, the Census Bureau is committed to closely following and studying these trends toward wireless communications.”

 Find more about the GAO evaluations and other analyses of the 2010 census at All Things Census.