Fig. 1From: Heat stroke admissions during heat waves in 1,916 US counties for the period from 1999 to 2010 and their effect modifiersAn example illustrating how control days were matched to heat wave days. July 30, 1999 was a heat wave day in Chicago. Five candidate control days occurred in the same county and same week of the year as this heat wave day. One of them was randomly selected and matched with the heat wave dayBack to article page