The following exercise was suggested by an article appearing in Public Health Reports [3|. A survey was taken to determine the incidence of HIV seroconversion among first-time blood donors at a blood center located in a large city. During a particular month, 180 first-time donors gave blood at the center. Of these, 175 were seronegative. A sample of 60 from the seronegative first-time donors was selected and given appointments to give blood again over the next 6 months. The following data were obtained from these persons.

From these data, estimate the annual incidence of HIV seroconversion among first-time blood donors at this center. Give 95% confidence intervals for this incidence rate.