The researchers looked at the frequency of antibiotic prescription in each of the three time frames. Then they compared antibiotic use in 1991–1992 with use in 1998–1999 for various conditions, including the common cold, sore throat, sinus infections, and bronchitis. Viruses usually cause these conditions, so antibiotics would be unlikely to help patients. The researchers paid particular attention to the use of broad-spectrum antibiotics.