The sixth quality criteria evaluated the case definition used. Of the 14 studies that reported incidence, 5 (36%) used the WHO case definition for pneumonia and 4 (26%) used the Chinese Medical Association guidelines case definition; 5 (36%) used physician diagnosis, which was considered less reliable than the other case definitions. The incidence estimates using the WHO case definition were generally higher than those using a physician diagnosis, so we report incidence estimates separately by case definition; there was no clear trend in mortality estimates based on case definition, so we did not separate these estimates by case definition. Five (33%) of 15 incidence studies and only 1 (4%) mortality study included chest x-ray in their case definition. Most of the mortality data relied on physician diagnosis (23 [82%] of 28 studies). One quarter (7 of 28) of the mortality studies were reports of deaths for children under 5 years of age collected through the national death surveillance program.