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 2011–2018 ranged from 24.6% to 33.5%. A more detailed description of the methods can be found at the National Immunization Survey Website.
Effect of Changes in NIS Sampling Method
In 2011, a 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. See full report pdf icon[PDF-137KB].
In 2018, the NIS sampling frame was changed to include only a cell phone sample (referred to as single cell-frame sampling). To estimate the effect of this sampling frame change on breastfeeding rates, we examined data from the 2016 and 2017 NIS surveys to estimate the difference in breastfeeding rates derived from the dual-frame and single cell-frame for infants born in 2015. See full report. pdf icon[PDF-1.07MB].
Key findings for the change to a cell phone-only sample:
- Among infants born in 2015, the change from dual frame to single cell-frame sampling of respondents had a minimal effect on national estimates of all 6 breastfeeding indicators examined (ever breastfed, breastfed at 6 and 12 months, exclusive breastfeeding through 3 and 6 months, and formula supplementation before 2 days among breastfed infants); national prevalence estimates differed by 0.2 percentage points or less.
- The sampling change had a slightly larger effect on estimates of breastfeeding at the state level, with some states showing higher rates and some showing lower rates from the dual-frame compared to the single cell-frame estimates. Across the 6 indicators examined, the magnitude of the difference ranged from 0.0 to 5.4 percentage points, and was less than 2.0 percentage points for about 90% of the state estimates.
- We caution against making comparisons of current single cell-frame estimates of breastfeeding rates to previous dual-frame estimates at the state level.