David Hemenway at the Harvard School of Public Health Injury Control Research Center, said that Kleck's estimates are difficult to reconcile with comparable crime statistics, are subject to a high degree of sampling error, and that "because of differences in coverage and potential response errors, what exactly these surveys measure remains uncertain; mere repetition does not eliminate bias." Kleck and Gertz responded to this criticism saying: "It is obvious to us that David Hemenway had no intention of producing a balanced, intellectually serious assessment of our estimates of defensive gun use (DGU). Instead, his critique serves the narrow political purpose of 'getting the estimate down,' for the sake of advancing the gun control cause."