Methods: Breastfeeding Rates
National Immunization Survey (NIS)
About the Survey
Each year, CDC’s National Center for Immunizations and Respiratory Diseases (NCIRD), in partnership with the National Center for Health Statistics, conducts the National Immunization Survey (NIS). The NIS uses random-digit dialing to survey households with children and teens. The telephone survey asks questions about childhood immunization. Beginning in July 2001 and continuing through December 2002, a subsample of respondents was asked about breastfeeding. Starting in January 2003, all respondents with children aged 19 to 35 months are asked the breastfeeding questions.
The Council of American Survey and Research Organizations response rates for landline sample of NIS years 2001–2017 ranged from 51.9% to 76.1%. Response rates for the cellular telephone sample of NIS years 2011–2017 ranged from 25.0% to 33.5%. A more detailed description of the methods can be found at the National Immunization Survey Website.
Effect of Adding a Cellular Telephone Sample
In 2011, cellular telephone sample of respondents was included in the NIS along with the landline telephone sampling frame used for all previous survey years (referred to as dual-frame sampling). To evaluate the impact on estimates of breastfeeding rates when the NIS added a cellular phone sample of respondents, CDC compared the differences in these estimates between the landline and dual-frame samples using data from the same birth cohort.
NOTE: This document does not include state tables.
- Adding a cell phone sample to the NIS survey had a minimal effect on national estimates of the percentage of infants ever breastfed and the percentage exclusively breastfed at 3 and 6 months. Dual-frame prevalence estimates for these indicators differed from landline frame estimates by less than one percentage point for infants born in 2010, ranging from 0.5 percentage points lower for exclusively breastfed at 3 months to 0.2 percentage points higher for ever breastfed and 0.8 percentage points higher for exclusively breastfed at 6 months.
- Adding a cell phone sample had a slightly larger effect on national estimates of the duration of any breastfeeding. Among infants born in 2010, the dual-frame estimates were lower than the landline frame estimates by 1.5 and 1.6 percentage points for breastfed at 6 months and at 12 months, respectively.
- Adding a cell phone sample also had an effect on estimates of breastfeeding at the state level, with some states showing higher rates and some showing lower rates. However, the magnitude of the difference was less than 5 percentage points for 75% of the state estimates for infants born in 2010. State breastfeeding estimates from the National Immunization Survey do fluctuate from year-to-year because of the small size of the state samples.
- Based on our findings, we caution against making comparisons of current dual-frame birth-year estimates of breastfeeding duration to more recent landline frame estimates (e.g., 2007 or 2008 birth years) when there was an increasing number of US children living in households with only cell phone service and the National Immunization Survey did not yet include a cell phone sample of respondents. We have less concern about comparing current dual-frame birth-year estimates to landline frame estimates from birth years earlier in the decade when few households were cell phone only.