Project Overview & FAQ

The Integrated Fertility Survey Series (IFSS) project will develop a harmonized dataset based on nationally-representative U.S. family and fertility surveys spanning the 1955 - 2002 period, including the 1955 and 1960 Growth of American Families (GAF); the 1965 and 1970 National Fertility Survey (NFS); and Cycles 1 - 6 (1973, 1976, 1982, 1988, 1995, 2002) of the National Survey of Family Growth (NSFG). This new dataset and its associated data tools will render easier and more efficient analyses across a 50-year period, yielding new insights into changes in fertility and the family. The IFSS project seeks to:

• develop a harmonized data file and accompanying metadata for use by researchers, graduate students, policymakers, and other parties interested in changes in fertility and the family;
• create, archive, and distribute clean, standardized electronic data files and documentation for the ten component surveys;
• equip IFSS studies with online data analysis tools to allow for quick analyses;
• offer complete documentation, tutorials, and other user support for those who want to use data from the harmonized or individual component files.

The IFSS will provide a national resource to enhance knowledge about relationship and family behaviors, reproductive health, contraceptive use, pregnancy, and other issues vital to understanding and tracking population trends. The project period runs from spring 2007 to spring 2012 and products will be released as they become available.

The harmonization part of this project is often the focus, but creating uniform data files and documentation for the 10 individual surveys was no small task. The project team would like to acknowledge the contributions of those who work on the NSFG at the National Center for Health Statistics. They provided a great deal of assistance with locating original documentation, sharing setup files where available, and general knowledge about the NSFG. Learn more about the NSFG.

Frequently Asked Questions (FAQ) About the Integrated Fertility Survey Series (IFSS) Project

What is the importance of the Integrated Fertility Survey Series?

Though researchers from a broad range of disciplines have produced a large body of research on patterns in families and fertility, the ability to make comparisons over time – a central task for understanding family change – has been constrained by difficulties in using multiple datasets to perform time-series analyses. Such difficulties include changes in respondent universe, weighting procedures, imputation protocols, question wording, and variable availability across studies. This is especially true when attempting to include surveys from the earlier years. The IFSS project attempts to address these limitations by developing a data set that allows for comparisons across longer periods than were previously feasible. The primary aim of the IFSS is to establish a harmonized set of data and documentation across ten nationally representative surveys of fertility and family. It is expected that harmonization of common variables across multiple surveys will allow researchers, policymakers, students, and other constituencies to make comparisons across time.

What is harmonization? Why harmonize variables?

Harmonization is a process by which variables are made comparable across survey years. Harmonization schemes must be developed for each IFSS variable so that comparisons can be made across time. In general, harmonization in the IFSS project involves combining, into a single variable, information covering comparable substantive ground but from different files in the original GAF, NFS, and NSFG data sets.

Differences in question text, sample design, and respondent universes further complicate the harmonization process. Challenges posed by this part of harmonization include varying universes by survey (and survey year) and question text. For example, the IFSS data set will contain a variable for ever having used contraception. In the 1970 National Fertility Survey, the universe of respondents of whom the question is asked contains all respondents in the sample. However, in the 1965 National Fertility Survey, the universe is smaller; it contains only respondents who were not pregnant at the time of the survey interview. For other variables, differences in universe are not observed. For example, respondent's age is asked of all respondents in all ten component surveys; therefore, respondent's age poses no universe problems to those using the IFSS data set. Given the subtle variation in variables across surveys, the IFSS staff must exercise care in evaluating variable comparability.

To assist users of the IFSS data, IFSS staff are developing comprehensive variable documentation that will address important comparability notes across variables and studies. Especially serious, for example, are comparability problems that are not evident from the coding structure, including alterations in the survey question wording and changes in the variable universe. Such documentation will include notes about changes in respondent universe, question design, data collection instrument design, and other information necessary for informed use of IFSS data. The IFSS project staff will seek to make transparent all decisions made in the harmonization process so researchers can choose whether to use the harmonized version or to create one of their own.

How are variables selected to be harmonized? Are all variables harmonized across all data sets?

With consultation from the IFSS advisory panel – composed of distinguished experts in fields as diverse as demography, economics, public health, survey methodology, and sociology – variables selected for inclusion in the IFSS data set will be identified on the basis of their expected interest to social science researchers, graduate students, policymakers, and other constituencies. Not all variables in all surveys will be harmonized. In most cases, variables selected for inclusion in the IFSS data set will have comparable equivalents across three or more surveys.

What are the next steps for the IFSS project?

Variables will be harmonized and released in stages. Sociodemographic variables will be the first set to be harmonized, and the IFSS staff expect that this first set will be publicly released during the summer of 2009. Other types of variables to be harmonized, in no specific order, include those related to children, union formation and dissolution, reproduction, contraception, attitudes, and other family and fertility-related behaviors.