Section 1.2 The National Survey of Family Growth
Since 1973 the U.S. Centers for Disease Control and Prevention (CDC) have conducted the National Survey of Family Growth (NSFG), which is intended to gather "information on family life, marriage and divorce, pregnancy, infertility, use of contraception, and men’s and women’s health. The survey results are used...to plan health services and health education programs, and to do statistical studies of families, fertility, and health."
You can read more about the NSFG at
cdc.gov/nchs/nsfg.htm.
We will use data collected by this survey to investigate whether first babies tend to be born late, and other questions. In order to use this data effectively, we have to understand the design of the study.
In general, the goal of a statistical study is to draw conclusions about a population. In the NSFG, the target population is people in the United States aged 15-44.
Ideally surveys would collect data from every member of the population, but that’s seldom possible. Instead we collect data from a subset of the population called a sample. The people who participate in a survey are called respondents.
The NSFG is a cross-sectional study, which means that it captures a snapshot of a population at a point in time. The NSFG has been conducted several times now; each deployment is called a cycle. We will use data from Cycle 6, which was conducted from January 2002 to March 2003.
In general, cross-sectional studies are meant to be representative, which means that the sample is similar to the target population in all ways that are important for the purposes of the study. That ideal is hard to achieve in practice, but people who conduct surveys come as close as they can.
The NSFG is not representative; instead it is stratified, which means that it deliberately oversamples some groups. The designers of the study recruited three groups — Hispanics, African-Americans and teenagers — at rates higher than their representation in the U.S. population, in order to make sure that the number of respondents in each group is large enough to draw valid conclusions. The drawback of oversampling is that it is not as easy to draw conclusions about the population based on statistics from the sample. We will come back to this point later.
When working with this kind of data, it is important to be familiar with the codebook, which documents the design of the study, the survey questions, and the encoding of the responses.
The codebook and user’s guide for the NSFG data are available from
cdc.gov/nchs/nsfg/nsfg_cycle6.htm.
