You have been asked to retrospectively sample records for a study for one of your pediatricians. She wants to examine a possible correlation between a child's immunization status (up-to-date or not) and the caregiver's type of insurance. You will need to come up with criteria for the immunization status and a key for the type of insurance. After getting a listing of the medical record numbers for the hospital's pediatric patients, you look at your options for choosing a random sample of 30 records to get the study started