Most gun control policies currently advocated in the United States (e.g., licensing, registration and one-gun-a-month) could be described as efforts to limit the supply of guns available in the illegal market. However, once such laws are enacted, it is important to demonstrate that they are effective.
With this mission in mind, researchers set out to show that Virginia's one-gun-a-month law has been effective in reducing the supply of guns on the illegal market -- a somewhat less than UNBIASED research premise.
Their data consisted of 14,606 firearms recovered by law enforcement before and after the enactment of the Virginia law. The researchers concluded that guns recovered after enactment of the law were 36 percent less likely to have come from Virginia than before the law. The conclusion was that Virginia's one-gun-a-month law is effective. BUT:
Ever heard of representative sampling? Not all recovered firearms are traced. The researchers never determined whether the firearms in this study are representative of all recovered firearms.
It's a good thing these researchers started out with a preconceived conclusion. Otherwise, I think their research would have ended up where it belonged... nowhere.