From the 2006 National Immunization Survey:
Was [child] ever breastfed or fed breast milk?
Using a computer-generated list, the NIS identifies households across the United States with children aged 19–35 months and interviews the person who is most knowledgeable about the child’s immunization status (“caregiver”). Survey years are combined to calculate breastfeeding statistics by year of child’s birth (cohort) instead of the year in which the participant was surveyed. To calculate breastfeeding indicators by year of child birth, data are combined across all relevant survey years. Because children are 19—35 months of age at the time of the parent interview, each survey year represents children born over three years. For example, breastfeeding data for children in the 2006 birth cohort are obtained from NIS 2007, 2008, and 2009.