By Lauren Musu-Gillette
EDITOR’S NOTE: This is the first in a series of blog posts about statistical concepts that NCES uses as a part of its work.
The National Center for Education Statistics (NCES) collects survey statistics in two main ways—universe surveys and sample surveys.
Some NCES statistics, such as the number of students enrolled in public schools or postsecondary institutions, come from administrative data collections. These data represent a nearly exact count of a population because information is collected from all potential respondents (e.g., all public schools in the U.S.). These types of data collections are also known as universe surveys because they involve the collection of data covering all known units in a population. The Common Core of Data (CCD), the Private School Survey (PSS) and the Integrated Postsecondary Education Data System (IPEDS) are the key universe surveys collected by NCES.
While universe surveys provide a wealth of important data on education, data collections of this magnitude are not realistic for every potential variable or outcome of interest to education stakeholders. That is why, in some cases, we use sample surveys, which select smaller subgroups that are representative of a broader population of interest. Using sample surveys can reduce the time and expense that would be associated with collecting data from all members of a particular population of interest.
Example of selecting a sample from a population of interest
The example above shows a simplified version of how a representative sample could be drawn from a population. The population shown here has 60 people, with 2/3 males and 1/3 females. The smaller sample of 6 individuals is drawn from this larger population, but remains representative with 2/3 males and 1/3 females included in the sample.
For instance, the National Postsecondary Student Aid Study (NPSAS), Baccalaureate and Beyond (B&B), and the Beginning Postsecondary Study (BPS) select institutions from the entire universe of institutions contained in the Integrated Postsecondary Education Data System (IPEDS) database. Then, some students within those schools are selected for inclusion in the study.
Schools and students are selected so that they are representative of the entire population of postsecondary institutions and students. Some types of institutions or schools can be sampled at higher rates than their representation in the population to ensure additional precision for survey estimates of that population. Through scientific design of the sample of institutions and appropriate weighting of the sample respondents, data from these surveys are nationally representative without requiring that all schools or all students be included in the data collection.
Many of the NCES surveys are sample surveys. For example, NCES longitudinal surveys include nationally representative data for cohorts of students in the elementary grades (Early Childhood Longitudinal Survey), the middle grades (Middle Grades Longitudinal Study), as well as at the high school (High School Longitudinal Study), and college levels (Beginning Postsecondary Students). The National Household Education Survey gathers information on parental involvement in education, early childhood programs, and other topics using household residences rather than schools as the population. The National Postsecondary Student Aid Survey gathers descriptive information on all college students and their participation in student aid programs. Additionally, characteristics of teachers and principals and the schools in which they teach are obtained through the Schools and Staffing Survey, and the National Teacher and Principal Survey.
By taking samples of the population of interest, NCES is able to study trends on a national level without needing to collect data from every student or every school. However, the structure and the size of the sample can affect the accuracy of the results for some population groups. This means that statistical testing is necessary to make inferences about differences between groups in the population. Stay tuned for future blogs about how this testing is done, and how NCES provides the data necessary for researchers or the public to do testing of their own.