Loftin C, McDowall D, Wiersema B, and Cottey TJ. Effects of Restrictive Licensing of Handguns on Homicide and Suicide in the District of Columbia. N. Engl J Med 1991; 325:1615-20.
methodological and conceptual errors:
- the apparent, temporary, and minuscule homicide drop occurred 2 years before the Washington DC law took effect
- the "interrupted time series" methodology as used by Loftin et al. has been invalidated
- the study used raw numbers rather than population-corrected rates -- not correcting for the 20% population decrease in Washington, DC during the study period or for the 25% increase in the control population -- exaggerating the authors' misinterpretations
- the study conveniently stopped as Washington, DC's overall homicide rate skyrocketed to 8 times the national average and the Black, male, teen homicide rate skyrocketed to 22 times the national average
- used a drastically dissimilar demographic group as control
- the authors virtually failed to discuss the role of complicating factors such as the crack cocaine trade and criminal justice operations during the study period
Loftin et al. attempted to show that Washington, DC's 1976 ban on new gun sales decreased murder.  Loftin and his co-authors, using tax money, produced "research" with several negating flaws that were ignored or overlooked by "peer review" and the editorial board of the New England Journal of Medicine -- perhaps a corollary of the editor's no-data-are-needed  policy.
Not only has the "interrupted time series" methodology as used by Loftin et al. has been invalidated,  but the temporary and minuscule homicide drop began during 1974, 2 years before the gun law. How could the law, even before its proposal, be responsible for the drop? Since homicidal maniacs and criminals could not clairvoyantly anticipate the law, other causalities should have been considered. The authors, however, side-stepped the question and dismissed non-gun causalities without any analysis whatsoever.
The study conveniently stopped as the Washington, DC homicide rate skyrocketed. If the gun freeze law, which has not changed, were responsible for the homicide drop, we would expect the "drop" to continue. If the "guns-cause-murder" theory is valid and if the gun freeze were effective, as "grandfathered" guns leave circulation (owner moves, dies, guns become unserviceable, etc.), the homicide rate should drop steadily. Quite the opposite is observed. The 1976 Washington, DC homicide rate before the law was 26.9 (derived from population  and homicide  statistics) and then tripled to 80.6 by 1991  despite or due to the law;
Justifiable and excusable homicides, including those by police officers, were treated the same as murders and were not excluded from the study. The study used raw numbers rather than population-corrected rates. This did not correct for the 20% population decrease in Washington, DC during the study period or for the 25% increase in the control population -- exaggerating the authors' misinterpretation. The study used the adjacent suburbs as a control group, an area with demographics drastically different from the study group.
The authors examined and allowed only a single cause interpretation -- guns are to blame. They offhandedly discarded any other possible explanation. They specifically ignored the role of the crack cocaine trade, FBI stolen property and Bureau of Alcohol, Tobacco and Firearms illegal weapon sting operations in progress during the study, and measures instituted during the study period that improved the efficiency of the Washington DC court system. They generally ignored the role of poverty and myriad other factors related to criminal violence.
Homicide has declined for every segment of American society except teenage and young adult inner-city residents. The Black teenage male homicide rate in Washington, DC is 227 per 100,000,  yet less than 7 per 100,000 for rural, middle-aged white men,  the US group for whom gun ownership has the highest prevalence.  If the "guns-cause-violence" theory is correct why does Virginia, the alleged "easy purchase" source of all those illegal Washington, DC guns, not have a murder rate comparable to DC? The "guns-cause-violence" theory founders. [See Graph 2: "Selected Homicide Rates Comparisons"]
Even in their responses to criticism,  the authors' intransigent bias is evident. Their position? If a drop in murder is discovered (or statistically contrived), gun control must receive the credit, but when attention was drawn to the failures of gun control and their study design, the skyrocketing murder rate must be credited to "other causes." Shall we examine gun control as science or religion? It appears that the faith of true believers is unshakable heedless of data and the scientific method.