I'm going to try to answer in clear language, avoiding the jargon of the statistician and, in some cases, the criminologist.
Briefly, we conduct trend analyses on the rates of homicides and suicides committed using firearms and other means. We conduct analyses in which we take existing trends into account. We know very well that the trend is downward. We also consider socio-economic circumstances. We know that firearms control is not the only factor that may influence the rates. Lastly, based on all these factors, we determine whether there was a drop in the rates of suicides and homicides committed using firearms before and after the implementation of Bill C-68. We repeat the same procedure for the rates of suicides and homicides committed by other means and, with the aid of statistical criteria, we create estimates.
Those estimates tell us whether there has been a significant decline in the rates of homicides and suicides committed by firearms following implementation of Bill C-68 and that, at the same time, the rates of homicides and suicides committed by other means did not increase following implementation of Bill C-68. This means that there is no so-called displacement effect. I would add that these methods are recognized as being the most rigorous in the scientific field.