Bucking national street violence trends, shootings and homicides decreased in Boston this year compared to last year, while gun arrests in the city have increased. Philip Cook, a public policy professor emeritus at Duke University, said in an e-mail that in contrast to Boston’s figures, the national count “has already matched the total for all of last year, which in turn was 26 percent higher than the previous year.” Whatever the cause, the reduced crime rates represent a positive story for a police department that has been buffeted by scandal throughout the year, from allegations of overtime fraud at an evidence warehouse to revelations that the department allowed an officer to continue to serve on the force for years after investigators determined in the mid-1990s he had more than…