The Thai trial, the largest HIV vaccine trial in history, took place over 3 years from 2003 to 2006. The trial recruited more than 16,000 volunteers in Thailand aged 18 to 30 and was designed to test a two-vaccine combination. It was largely financed by the U.S. government and endorsed by eleven international review bodies. Roughly half were randomly picked to receive the vaccine, while the other half got dummy shots. When the trial was completed, a relatively small number of people became infected with HIV 51 of the 8,197 people given the vaccine, and 74 of the 8,198 who re- ceived dummy shots (The New England Journal of Medicine, Oct, 20, 2009). Is this difference statistically significant?