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Does D.C. really have a 94% homicide closure rate?

| 3 Comments
No.  Cheryl W. Thompson has this story in the WaPo.

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.

3 Comments

Probably the right way to do it is to have a cut off date for recent crimes.

The problems are (1) the shorter the cutoff date, the more you understate the real closure rate; (2) the longer the cutoff date, the less useful the number is by the time you finally get it. If you looked at the closure rate for homicides committed 20 years ago, we might say we are reasonably confident that we have solved all or nearly all of the homicides that are going to be solved, but how useful is the number from 20 years ago?

And even that confidence might be misplaced. As San Joaquin County, California begins the grim task of DNA-identifying all the bones dug out of the well in Linden, the clearance rates from years as far back as 1984 will go up.

Murder is a gruesome business. There is little doubt about that.

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