NIS Survey Methods
Each year, the CDC National Immunization Program, in partnership with CDC's National Center for Health Statistics, 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. Beginning in the 4th quarter of 2010, a cell-phone sample of respondents was included in the NIS along with the landline telephone sampling frame used for all previous survey years. However, breastfeeding rates continue to be based only on the landline telephone sample. We are currently evaluating the effect of the cell phone sample addition to better understand the effect on breastfeeding rates. We plan to report rates on the basis of dual- frame sampling (landline and cell-phone samples) in August 2014 along with a description of the effect on trends in breastfeeding. The Council of American Survey and Research Organizations response rates for landline sample of NIS years 2001–2012 were consistently above 60% (range 61.6%–76.1%). A more detailed description of the methods can be found at the National Immunization Survey Web site.
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 prior to 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 2001–2012
|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?|
Analysis of Breastfeeding Practice Data by Year of Child Birth
We combined all available survey years to calculate breastfeeding indicators by year of child birth (Smith et al., 2006). Because children are 19–35 months of age at the time of the parent interview, each survey year represents children born over three years (Table A2). Breastfeeding rates among children in a birth year are released when approximately 2/3rds of the children born in that year have been surveyed. The rates are labeled provisional until they are replaced the following year with final rates based on all children in the birth year. All analyses are conducted using statistical software that accounts for complex sample design.
Smith PJ, Zhao Z, Wolter KM, Singleton JA, Nuorti JP. Age-period-cohort analyses of public health data collected from independent serial cross-sectional complex probability sample surveys. Paper presented at the Joint Statistical Meeting: 2006; Seattle, WA. http://www.amstat.org/Sections/Srms/Proceedings/y2006/Files/JSM2006-000537.pdf - [PDF-125k]