I applied the capture-recapture method to estimate the World Trade Center tower population at the time of the September 11, 2001, terrorist attacks. Available lists helped identify 8965 survivors and 2152 confirmed casualties. The capture-recapture model suggested that an additional 4435 survivors were present, putting the total count of all present at 15552 (95% confidence interval=15216, 15897). An accurate estimate represents the potential number at risk for trauma as a result of direct exposure to the events of the day. On September 11, 2001, 2 airplanes crashed into the World Trade Center towers in New York City as part of a terrorist attack. In addition to the loss of life, survivors were exposed to hazardous dust and debris from the collapse of the buildings, and many people endured psychological trauma. No one has definitively determined the number of persons present in the 2 towers at 8:46 AM on that day when the first airplane crashed into the North Tower, and no one may ever be able to do so. Several estimates have beenmade previously, but none made use of capture-recapture methods, which are useful in estimating population size when enumeration by more-direct methods is not feasible. My objective was to apply the capture- recapture method to 3 list sources of individuals determined to be present inside the World Trade Center towers on September 11 to estimate the number present when the first airplane struck. This number represents a population at risk for long-term health effects as a result of direct exposure to the events of the day.
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