But an examination of District homicides found that the department's closure rate is a statistical mishmash that makes things seem much better than they are. The District had 108 homicides last year, police records show. A 94 percent closure rate would mean that detectives solved 102 of them. But only 62 were solved as of year's end, for a true closure rate of 57 percent, according to records reviewed by The Post.
D.C. police achieved the high closure rate last year by including about 40 cases from other years that were closed in 2011.
Similar problems arise in many contexts when we insist on shoehorning things into a single calendar year that don't really fit in one year. That is one of the things that make income tax law so complex, for example.
If homicide rates were steady, the ones unsolved this year and solved later might roughly balance out those committed in prior years and solved in this one. But homicide rates have generally declined in most cities, including D.C., so the pool of unsolved cases from prior years is proportionately larger than the unsolved cases from the year just ended.
In the end, there is no way to come up with a single "closure rate" number for a single year. Reality is messier than that.