I've been reading here..... http://gunsandcrime.org/stats.html

People are tempted to eliminate some of the competing alternative explanations based on logic. However, doing so demands a perfect understanding of the relationships between the factors being evaluated. In other words, the "logic" must be correct. Relying on logic to eliminate competing explanations typically results in erroneous interpretation of study results, especially if the process is being attempted by someone who knows little about the topic (like gun controllers who have never owned a gun deciding about gun matters impacting crime, violence, accidents, etc.). A study that finds that there is a high gun ownership rate in a time and place where there is a high murder rate, for example, might have such findings in part because a high murder rate causes people to acquire guns to protect themselves, their families and their neighbors. It is naive or dishonest to claim on the basis of such results that high gun ownership causes high murder rates.

Another fact that suggests that one factor is the cause of the other is that the second factor does not ever change until introduction or forced change of the first. If violent crime rates in a state start dropping faster after a right-to-carry law goes into effect in that state, after accounting for any other changes that went into effect at the same time, this would be very strong indication that the right-to-carry law causes crime reduction. On the other hand, a Washington, DC crime rate reduction trend that starts two years before a firearm ban in Washington, DC pretty much proves that the crime rate reduction was not caused by the ban.

Hey, Brady bunch....read this.