Death certificates are important public health documents. They help epidemiologists understand leading causes of deaths and how they are changing. They power big studies of what killed us in the past — and what kills us now. And, according to a new Center for Disease Control study, about a third of them may be wrong. Columbia University's Barbara A. Wexelman led a survey of 521 resident physicians in New York City. About one-third of those doctors completed more than 11 death certificates in the past year, making them pretty familiar with how the system works. "Only one-third of the respondents," Wexelman and her team found, "believed the current system accurately documents correct cause of death."