By the afternoon of Friday, August 22, news reports showed that Washington, D.C. had gone nearly ten days without a homicide. Assuming that there were no homicides for the rest of yesterday, and we can find no record of that, there would have been ten days without a homicide in D.C. But since we are not certain, we will report the estimated probabilities for D.C. going for both nine and ten days without a homicide.
How likely is it that nine straight homicide-free days happened by chance? Using the homicide rate from the first seven months of 2025, the probability comes out to just 1.5%—well below the 5% threshold typically used to mark a result as statistically significant.
Data for 2024
Washington, D.C. recorded 187 homicides in 2024, a daily rate of 0.5137.
Using a Poisson distribution, the probability of no homicides in a given day is: 0.598. So, there’s about a 59.8% chance of a single day passing with zero homicides. Assuming for simplicity that each day is independent of all others, the probability of nine no-homicide days = (0.598)**9 = 0.0098 or 0.98%.
If this can go for ten days, the probability is 0.58%. If it can go for 14 days, it is 0.075%
Data for the first seven months of 2025
Washington, D.C. recorded 99 homicide for the first seven months of 2024, a daily rate of 0.467.
Using a Poisson distribution, the probability of no homicides in a given day is: 0.627. So, there’s about a 62.7% chance of a single day passing with zero homicides. Assuming that each day is independent of all others, the probability of nine no-homicide days = (0.627)**9 = 0.015 or 1.5%.
If this can go for ten days, the probability is 0.94%. If it can go for 14 days, it is 0.145%
