An epidemiologic investigation conducted on January 1st, 2009 identified a population of 1,000 individuals among whom 4 were found to have the disease under study on this date. During the year of the study, 6 additional new cases were found. Among the total of 10 cases, there were 6 deaths during the year. For the 10 cases, the diagram indicates the time of case recognition, periods of observation during the study, and vital status at the time of the termination of observation. A vertical bar at the start of the diagram (patients 1, 2, 3, 4) indicates that the start of the disease had occurred before the study began. Assume that the 990 remaining individuals in the study did not become ill or die during the year of observation.
What is the person-time at risk in this study?