Incorrect Number. The Medicine by the Numbers, “Valproate for Adult Migraine Prophylaxis” (November 1, 2016, online only) incorrectly defined the Study Population inclusion criteria as “Patients 16 years or older with episodes of migraine headache on at least 15 days of every month” rather than “Patients 16 years or older with episodes of migraine headache on fewer than 15 days of every month.” This error was repeated in the fourth sentence of the Narrative section, which should have read, “Ten randomized trials enrolled 2,296 patients 16 years or older with migraines occurring on fewer than 15 days of every month.” The online version of this department has been corrected.