Why It's Nearly Impossible to Count Police Killings

Less than two weeks after the non-indictment of Officer Darren Wilson in Ferguson, a New York grand jury is poised to decide whether a white police officer should be charged in the death of a young black man who was put in a choke hold. Protests continue over police killings but how many are there?  An investigation by the Wall Street Journal has revealed it's almost impossible to tell.  Rob Barry worked on the story.