NIS Survey Methods

Each year, CDC’s National Center for Immunizations and Respiratory Diseases (NCIRD) conducts the National Immunization Survey (NIS). The NIS uses random-digit dialing to survey households with children aged 19–35 months. The telephone survey asks questions about childhood immunization. Beginning July 2001 and continuing through December 2002, a sample of respondents was asked about breastfeeding using the original questions listed in Table A1. Starting January 2003, all respondents to the telephone survey were asked the breastfeeding questions. The Council of American Survey and Research Organizations response rates for landline sample of NIS years 2001–2016 ranged from 55.7% to 76.1%. Response rates for the cellular telephone sample of NIS years 2011–2016 ranged from 25.2% to 33.5%. A more detailed description of the methods can be found at the National Immunization Survey website.

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 breastfeeding rates when the NIS added a cellular phone sample of respondents, CDC compared the differences in breastfeeding rates between the landline and dual-frame samples using data from the same birth cohort.

Key findings of study comparing breastfeeding rates from landline and dual-frame samples

• Adding a cell phone sample to the NIS had a minimal impact on national estimates of the percent of infants ever breastfed and the percent 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 breastfeeding and 0.8 percentage points higher for exclusively breastfed at 6 months.
• Adding a cell phone sample had a slightly larger impact 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. State breastfeeding rates estimated from NIS do fluctuate from year to year due to 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 to more recent landline frame estimates (e.g. 2007 or 2008 birth year) when there was an increasing number of U.S. children living in households with only cell phone service and the NIS did not yet include a cell phone sample of respondents. We have less concern about comparison of current dual-frame birth year estimates to landline frame estimates from birth years earlier in the decade when few households were cell phone only.

The full study report is available at http://www.cdc.gov/breastfeeding/data/nis_data/estimation-bf-rates.htm

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Wording of the Breastfeeding Questions

The breastfeeding questions used in the NIS were modified in 2004 and 2006 (Table A1). In January 2004, the third question was modified slightly to remove “or water” from the first part of the question and add it to the list of items other than breast milk. Additionally, the interviewers were instructed to always read the clarification of foods and liquids to consider. Our examination of the influence of this question change on the rate of exclusive breastfeeding at 3 and 6 months of age revealed a minimal effect.

In 2006, additional changes were made to two of the NIS breastfeeding questions (Table A1). These changes were on the basis of cognitive testing results for the questions used before 2006. Question 2 was revised to read “How old was [child’s name] when [child’s name] completely stopped breastfeeding or being fed breast milk?” The wording change on question 2 resulted in a minimal effect on our reported duration of breastfeeding. Question 3, “How old was [child] when s/he was first fed something other than breast milk?” was asked as two separate questions in 2006, which yielded significantly lower estimates of exclusive breastfeeding. Because of this large effect, we only show trends of exclusive breastfeeding and formula supplementation of breast milk for children whose caregivers were interviewed after December 31, 2005.

The 2006 breastfeeding questions continue to be used in the NIS survey.

Table A1. Changes in breastfeeding questions, National Immunization Survey 2001–2016
Questions 2001–2003	Questions 2004–2005	Questions 2006–present
1. Was [child] ever breastfed or fed breast milk?	1. Was [child] ever breastfed or fed breast milk?	1. Was [child] ever breastfed or fed breast milk?
2. How long was [child] breastfed or fed breast milk?	2. How long was [child] breastfed or fed breast milk?	2. How old was [child’s name] when [child’s name] completely stopped breastfeeding or being fed breast milk?
3. How old was [child] when s/he was first fed something other than breast milk or water? [If respondent hesitates, add: This includes formula, juice, cow’s milk, sugar water, solid foods or anything else.]	3. How old was [child] when s/he was first fed something other than breast milk? This includes formula, juice, cow’s milk, water, sugar water, solid foods or anything else.	3. How old was [child’s name] when (he/she) was first fed formula?
		4. This next question is about the first thing that [child] was given other than breast milk or formula. Please include juice, cow’s milk, sugar water, baby food, or anything else that [child] may have been given, even water. How old was [child’s name] when (he/she) was first fed anything other than breast milk or formula?

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Analysis of Breastfeeding Practice Data by Year of Child Birth

We combine survey years to calculate breastfeeding indicators by year of child birth (Smith et al., 2006). Because children were 19–35 months of age at the time of the parent interview through NIS survey year 2010 and 19-35 months of age any time during the survey year quarter starting with the 2011 NIS survey, each survey year represents children born over three years (see Table A2).

For each birth year, we estimate the percentage of infants in the following categories: “ever breastfed,” “breastfed at 6 months,” “breastfed at 12 months,” “exclusively breastfed through 3 months,” and “exclusively breastfed through 6 months.” whose mothers started breastfeeding.

• “Ever breastfed” is estimated by the question: “Was [child] ever breastfed or fed breast milk?”
• “Breastfeeding duration” is estimated by the question: “How old was [child] when he/she completely stopped breastfeeding or being fed breast milk?”
• Because exclusive breastfeeding is defined as ONLY breast milk (no solids, water, or other liquids); the duration of exclusive breastfeeding is estimated by the two survey questions about age, including the age of the child when he/she was first fed formula, and the age of the child when he/she was first fed anything other than breast milk or formula (including water).

We also calculate the percentage of breastfed infants who are supplemented with infant formula before they are 2 days, 3 months, and 6 months old. The rates of formula supplementation (with or without other supplementary liquids or solids) before 2 days, 3 months, and 6 months are calculated among infants who are breastfeeding at each respective age,
whereas the breastfeeding initiation, duration, and exclusivity rates are calculated among all infants surveyed.

Breastfeeding rates among children in a birth year are released when approximately two thirds of the children born in that year have been surveyed. In the past, the rates were labeled provisional until they were replaced the following year with final rates based on all children surveyed in the birth year. Starting from 2010 births for landline rates (2013 CDC Breastfeeding Report Card) and 2009 births for dual frame rates (2014 NIS surveillance report), CDC now reports one final rate based on 2 survey years only. This is because adding a third year of survey data to a birth cohort has little impact on the breastfeeding rate and results in a margin of error that is only about 20% smaller than with two survey years. All analyses are conducted using statistical software that accounts for complex sample design.

Table A2. Years of breastfeeding data from National Immunization Survey used to calculate breastfeeding rates for each birth cohorta
NIS survey year	Birth mon/yr represented	Birth Year 2000	Birth Year 2001	Birth Year 2002	Birth Year 2003	Birth Year 2004	Birth Year 2005	Birth Year 2006	Birth Year 2007	Birth Year 2008	Birth Year 2009	Birth Year 2010	Birth Year 2011	Birth Year 2012	Birth Year 2013	Birth Year 2014
2002	1/99–5/01	X	X
2003	1/00–5/02	X	X	X
2004	1/01–5/03		X	X	X
2005	1/02–5/04			X	X	X
2006	1/03–5/05				X	X	X
2007	1/04–5/06					X	X	X
2008	1/05–5/07						X	X	X
2009	1/06–5/08							X	X	X
2010	1/07–5/09								X	X
2011	1/08–5/10									X	X	X
2012	1/09–5/11										X	X	X
2013	1/10–5/12												X	X
2014	1/11–5/13													X	X
2015	1/12-5/14														X	X
2016	1/13-5/15															X
aStarting with the 2010 birth cohort, CDC includes only the FIRST two survey years of data for annual dual-frame estimates of breastfeeding rates. However, the estimations for the 2009 birth cohort from the dual-frame samples were recalculated using the LAST two survey years of data because the cellular telephone sample was not added until 2011.

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