>> the data does not address how many of those deaths were deemed to be justified. what does this data tell us? >> s >> reporter: this study highlights the need for more comprehensive and public data system on police violence because you can't make change without all the facts. particularly because this data shows again as you mentioned how much people of certain races and ethnicities are impacted disproportionately by that violence. my colleague broke down the study and found researchers looked at a 40-year period in the u.s. and determined more than half of all deaths found to be due to police violence are underreported or misclassified in a government database where they can be ak sessioned publicly. a closer look at those numbers show the mortality rate over the four decades was 3 1/2 times higher among the black population and 1.8% more for hispanics.