Data are from the Virginia Department of Health Immunization Survey of Kindergarten, Head Start and Day Care programs. The statistical function known as probability proportional to size is used to select assessment sites. This function provides all students, regardless of geographic location, with an equal chance of being selected. Data collected by district health department staff are forwarded to the central office where they are imported into the Clinic Assessment Software Application (CASA). CASA analyzes the data taken from the student immunization records and provides vaccine coverage rates at school entry, retrospectively at 2 years of age and produces a listing of students with medical and religious exemptions to immunization.