UPDATED: John Lott Tries To Substantiate His Debunked Assertions By Repeating Them
John Lott responded  to a post  here at Media Matters that debunked his assertions that, "Virtually no criminal guns are obtained from gun shows," and that "Background checks do not stop criminals from getting guns."
It is apparent from his history that John Lott does not understand the basic concepts of science ,statistics  or ethics , so it is not surprising that his latest post offers as "proof" only a reassertion of the debunked claims.
First is his attempt to reassert that background checks do not stop criminals from getting guns:
LOTT: "Background checks do not stop criminals from getting guns." These guys at Media Matters are pretty dense because they seem to believe that these initial denials are stops that involve criminals when in fact they are almost all false positives. Over 99.9 percent of those purchases initially flagged as being illegal under the law were later determined to be misidentified. Take the numbers for 2008 (http://www.ncjrs.gov/pdffiles1/bjs/231052.pdf ), the latest year for which data are available. The 78,906 initial denials resulted in only 147 cases involving banned individuals trying to purchase guns.
In our original piece we used FBI data  that shows that the National Instant Criminal Background Check System (NICS) has in fact stopped 800,000 sales since 1998-- more than 600,000 of which involved convicted criminals.
Lott asserts that we must be pretty dense to think that these 800,000 instances "involve criminals." He states that "[o]ver 99.9 percent of those purchases initially flagged as being illegal" were later determined to be misidentified.
Lott is either willfully or ignorantly misunderstanding the background check system. The federal background check system prohibits certain buyers , such as convicted felons, anyone convicted of a crime of domestic violence, and other categories. The NICS system is set up to prohibit certain buyers from obtaining weapons, during the course of the background check sometimes the NICS system identifies persons who are then prosecuted.
M. Kristen Rand, Legislative Director for the Violence Policy Center explains further:
RAND: The act of trying to buy a gun when you are in a prohibited category is not a crime per se. The bottom line is that when Lott says "The 78,906 initial denials resulted in only 147 cases involving banned individuals trying to purchase guns," he is not accurately describing the category.
The 78,906 represents the universe of denials evaluated for referral for potential prosecution. Of those, 5,573 were referred to ATF field divisions for further review. Ultimately, the field offices referred 147 cases to prosecutors. This has absolutely nothing to do with whether the NICS accurately identified the 1 percent of people denied for being in a prohibited category, e.g. the system identifies that a buyer has a felony record and the transfer is denied. That person is a "banned individuals trying to purchase guns," as Lott describes it. The fact that that person is not later singled out for prosecution is irrelevant to the fact that he is in fact a prohibited purchaser.
Bottom line is that the NICS system and the background check system in place before NICS prevented1.9 million  attempted gun purchases by prohibited persons, convicted felons, domestic violence offenders and other prohibited groups. So yes, in fact background checks do keep guns out of the hands of criminals.
The second point that Lott tries to make true by repeating is:
LOTT: Virtually no criminal guns are obtained from gun shows. The BJS report "Firearms Use by Offenders" provides a survey of criminals convicted of gun offenses and it shows that 0.7 percent of criminals obtained their guns from gun shows.
The report  that Lott uses is a survey of prisoners serving sentences in a State or Federal prison. It is true that only 0.7 percent of respondents stated that they bought their guns at gun shows, but 81.6 percent of respondents stated that they bought their guns from a "family or [a] friend" or from a "street/illegal source." The survey has no data on where those 81.6 percent of firearms were originally purchased. Unless one was trying to make a political point by cherry picking data an actual researcher would need to go to another source to find meaningful data on the full role gun shows actually play in arming criminals.
The role of gun shows in illegal trafficking is extensively researched by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). According to the 2000 ATF report, Following the Gun: Enforcing Federal Firearms Traffickers :
Gun shows were a major trafficking channel, involving the second highest number of trafficked guns per investigation (more than 130), and associated with approximately 26,000 illegally diverted firearms. The investigations involved both licensed and unlicensed sellers at gun shows.
The same report states that gun shows account for 14 percent of all firearms identified in ATF trafficking investigations. A 2007 ATF report  on the results of investigative operations that included gun shows between May 2004 and August 2005 states:
Although the number of investigative operations at gun shows was low, the operations resulted in multiple arrests convictions, and firearm seizures.
As a result of the 202 investigative operations undertaken at 195 gun shows, ATF made 121 arrests that resulted in 83 convictions. (Some cases are still pending, so their final disposition is unknown.) Additionally, ATF seized 5,345 firearms during these investigative operations.
John Lott's apparent disregard for scientific rigor and allegations that he fabricated results have been well documented, here , here , here , here  and here . But, the most eloquent evisceration of his integrity comes from Michelle Malkin who wrote  about one of his many controversies:
MALKIN: The most disturbing charge, first raised by retired University of California, Santa Barbara professor Otis Dudley Duncan and pursued by Australian computer programmer Tim Lambert, is that Lott fabricated a study claiming that 98 percent of defensive gun uses involved mere brandishing, as opposed to shooting. When Lott cited the statistic peripherally on page three of his book, he attributed it to "national surveys." In the second edition, he changed the citation to "a national survey that I conducted."
Lott claims to have lost all of his data due to a computer crash. He financed the survey himself and kept no financial records. He has forgotten the names of the students who allegedly helped with the survey and who supposedly dialed thousands of survey respondents long-distance from their own dorm rooms using survey software Lott can't identify or produce. Assuming the survey data was lost in a computer crash, it is still remarkable that Lott could not produce a single, contemporaneous scrap of paper proving the survey's existence, such as the research protocol or survey instrument.
UPDATE 3/31/2011: from the author of the report titled , "Enforcement of the Brady Act, 2008: Federal and State Investigations and Prosecutions of Firearms Applicants Denied by a NICS check in 2008.", which Lott cites to justify the NICS inaccuracy claim.
You [Media Matters] asked me if it was accurate for an article to state that "Over 99.9 percent of purchases initially flagged as being illegal under the [Brady] law were 'false positives' - law-abiding citizens incorrectly identified as banned individuals." That statement cannot be supported by statistics on Brady Act background checks that have been collected since 1999.
A person whose firearm transfer application is denied by a background check pursuant to the Brady Act may appeal the denial. Statistics on appeals of denials have been collected since 1999 in a series of publications entitled Background Checks for Firearm Transfers, and are summarized in Trends for Background Checks for Firearm Transfers, 1999-2008. During that ten year period, nearly 1.5 million firearm transfer applications were denied and nearly 237,000 of those denials were appealed. Over 86,000 appeals resulted in a reversal of the denial, which would calculate to about 6% of the 1.5 million denials. Some of the reversals may have occurred because a person was mistakenly identified as being prohibited; however, no data is available on the reasons for reversals. The URL for Trends for Background Checks for Firearm Transfers, 1999-2008 ishttp://www.ncjrs.gov/pdffiles1/bjs/grants/231187.pdf .