Overview and Methods for the Youth Risk Behavior Surveillance System — United States, 2021

National YRBS Methodology

Overview

The national YRBS is conducted biennially, typically during the spring (January–June) of odd-numbered years among students in grades 9–12 enrolled in U.S. public and private schools. However, the 2021 national YRBS administration was postponed until fall (September–December) 2021 because of the COVID-19 pandemic and the shift to virtual and hybrid school instructional models and ongoing school closures during spring 2021. Biennial administration of the YRBS allows CDC to assess temporal changes in risk behaviors among the U.S. high school population. The national YRBS provides comparable data across survey years and allows state and local entities that conduct their own YRBSs to compare risk behaviors of their youths with those at the national level. A nationally representative sample of schools and a random sample of classes within those schools are selected to participate.

Questionnaire

In 2021, the national YRBS questionnaire consisted of 99 questions. Of those, 87 questions were included in the standard questionnaire* used by sites. Twelve questions were added to the standard questionnaire that reflected areas of interest for CDC and other partners. As in all cycles, both the previous year’s standard questionnaire and additional national-only questions were revised to include measurement of emerging and prevailing risk behaviors among high school students. Subject matter experts from CDC and elsewhere proposed changes, additions, and deletions to the questionnaire. Further refinements to the questionnaire were made based on feedback from cognitive testing. During this process the sexual identity question was modified (Table 1). In addition, in 2021, the national YRBS questionnaire was offered for the first time in English and Spanish.

All questions, except those assessing height, weight, and race, were multiple choice, with a maximum of eight mutually exclusive response options and only one possible answer per question. Most of the 2021 survey questions underwent test-retest analysis and demonstrated good reliability (13,14). The wording of each question, including recall periods, response options, and operational definitions for each variable, are available in the 2021 YRBS questionnaire and data user’s guide. (YRBSS data and documentation are available at https://www.cdc.gov/healthyyouth/data/yrbs/data.htm.)

In accordance with guidance from subject matter experts, response options for the sexual identity question were updated for the 2021 YRBS to include the following new categories: “I am not sure about my sexual identity (questioning),” “I describe my sexual identity in some other way,” and “I do not know what this question is asking.” As a result, beginning in 2021, YRBS can provide data for LGBQ+ students, as opposed to only lesbian, gay, or bisexual students.

Sampling

The 2021 YRBS sampling frame consisted of all regular public schools (including charter schools), parochial schools, and other private schools with students in at least one of grades 9–12 in the 50 U.S. states and the District of Columbia. Alternative schools, special education schools, schools operated by the U.S. Department of Defense or the Bureau of Indian Education, and vocational schools serving students who also attended another school were excluded. Schools with ≤40 students enrolled in grades 9–12 also were excluded. The sampling frame was constructed from data files obtained from MDR (formerly Market Data Retrieval) and the National Center for Education Statistics (NCES). NCES data sources included the Common Core of Data (https://nces.ed.gov/ccd) for public schools and the Private School Survey (https://nces.ed.gov/surveys/pss) for private schools. The YRBS sample size was increased in 2021 in anticipation of lower response rates resulting from the COVID-19 pandemic and to obtain a large enough sample size for the desired precision.

A three-stage cluster sampling design was used to produce a nationally representative sample of students in grades 9–12 who attend public and private schools. The first-stage sampling frame comprised 1,257 primary sampling units (PSUs), which consisted of entire counties, groups of smaller adjacent counties, or parts of larger counties. PSUs were categorized into 16 strata according to their metropolitan statistical area status (i.e., urban or nonurban) and the percentages of Black and Hispanic or Latino (Hispanic) students in each PSU. Sixty of the 1,257 PSUs were sampled with probability proportional to overall school enrollment size for that PSU. For the second-stage sampling, secondary sampling units (SSUs) were defined as a physical school with grades 9–12 or a school created by combining nearby schools to provide all four grades. From the 60 PSUs, 180 SSUs were sampled with probability proportional to school enrollment size. To provide adequate coverage of students in small schools, an additional 20 small SSUs were selected from a subsample of 20 of the 60 PSUs. These 200 SSUs corresponded to 209 physical schools. The third stage of sampling comprised random sampling of one or two classrooms in each of grades 9–12 from either a required subject (e.g., English or social studies) or a required period (e.g., homeroom or second period). All students in sampled classes who could independently complete the survey were eligible to participate. Schools, classes, and students that refused to participate were not replaced.

Data Collection Procedures

Institutional review boards at CDC and ICF, the survey contractor, approved the protocol for the YRBS. Data collection was conducted consistent with applicable federal law and CDC policy.^† Survey procedures were designed to protect students’ privacy by allowing for anonymous participation. Participation was voluntary, and local parental permission procedures were followed before survey administration. During survey administration, students completed the self-administered questionnaire during one class period (approximately 45 minutes) and recorded their responses on a computer-scannable booklet.

Response Rates and Data Processing

For the 2021 YRBS, 17,508 questionnaires were completed in 152 schools. The national data set was cleaned and edited for inconsistencies. Missing data were not statistically imputed. A questionnaire failed quality control when <20 responses remained after editing or when it contained the same answer to ≥15 consecutive questions. Among the 17,508 completed questionnaires, 276 failed quality control and were excluded from analysis, resulting in 17,232 usable questionnaires. The school response rate was 72.7%, the student response rate was 79.1%, and the overall response rate (i.e., [student response rate] x [school response rate]) was 57.5%.

Race and ethnicity were ascertained from two questions: 1) “Are you Hispanic or Latino?” (with response options of “yes” or “no”) and 2) “What is your race?” (with response options of “American Indian or Alaska Native [AI/AN],” “Asian,” “Black or African American [Black],” “Native Hawaiian or other Pacific Islander [NH/OPI],” or “White”). (Persons of Hispanic or Latino [Hispanic] origin might be of any race but are categorized as Hispanic; all racial groups are non-Hispanic.) For the second question, students could select more than one response option. For this report, students were classified as Hispanic or Latino and are referred to as Hispanic if they answered “yes” to the first question, regardless of how they answered the second question. For example, students who answered “no” to the first question and selected only Black or African American to the second question were classified as Black or African American and are referred to as Black. Likewise, students who answered “no” to the first question and selected only White to the second question were classified and are referred to as White. Race and ethnicity were classified as missing for students who did not answer the first question and for students who answered “no” to the first question but did not answer the second question. Students who selected more than one response option to “What is your race?” were classified as multiracial. Further, to meet the needs of an increasingly diverse population, CDC implemented modified suppression criteria for the YRBSS in 2021, allowing for increased data representation from students of diverse racial and ethnic groups. Previously, estimates with a denominator of <100 were suppressed; however, many of these estimates were found to be statistically reliable according to criteria set forth by CDC’s National Center for Health Statistics (15). Guided by these criteria, and in consideration of criteria used for other national surveillance systems, YRBS estimates with a denominator of <30 were suppressed in all years.

To obtain a sufficient sample size for analyses of health-related behaviors by sexual orientation (sexual identity and sex of sexual contacts), students were divided into groups (Table 1). Students who had no sexual contact were excluded from analyses related to sexual behaviors. Female students who had sexual contact with only females were excluded from analyses on condom use and dual use of condoms and birth control, and male students who had sexual contact with only males were excluded from analyses on dual use of condoms and birth control.

Weighting

A weight based on student sex, race and ethnicity, and grade was applied to each record to adjust for school and student nonresponse and oversampling of Black and Hispanic students. The overall weights were scaled so that the weighted count of students equals the total sample size, and the weighted proportions of students in each grade match the national population proportions. Therefore, weighted estimates are nationally representative of all students in grades 9–12 attending U.S. public and nonpublic schools.

Analytic Methods

Findings presented in this MMWR supplement are derived from analytic procedures similar to what is described in this overview report. For more information regarding the detailed analyses presented in this supplement (e.g., variables analyzed, custom measures, and data years), see the methods section in each individual report.

All statistical analyses were conducted using SAS-callable SUDAAN (version 11.0.3; RTI International) to account for the complex sampling design and weighting. In all reports, prevalence estimates and CIs were computed for variables used in those reports. Prevalence estimates where the denominator was <30 were suppressed. Pairwise differences between groups (e.g., sex, race and ethnicity, grade, sexual identity, and sex of sexual contacts) were determined using t-tests with Taylor series linearization. Pairwise differences were considered statistically significant if the t-test p value was <0.05. Chi-square tests were used to examine comparisons between risk behaviors and experiences by demographic and behavioral characteristics (race and ethnicity, grade, sexual identity, and sex of sexual contacts). Chi-square tests were considered statistically significant if the p value was <0.05.

In reports that analyzed temporal trends, logistic regression analyses were used to examine linear and quadratic changes in estimates, controlling for sex, grade, and racial and ethnic changes over time. A p value of <0.05 associated with a regression coefficient was considered statistically significant. Linear and quadratic time variables were treated as continuous and were coded by using orthogonal coefficients calculated with PROC IML in SAS (version 9.4; SAS Institute). A minimum of 3 survey years was required for calculating linear trends, and a minimum of 6 survey years was required to calculate quadratic trends. Separate regression models were used to assess linear and quadratic trends. When a significant quadratic trend was identified, Joinpoint (version 4.9; National Cancer Institute) was used to automate identification of the year when the trend changed. Regression models were used to identify linear trends occurring before and after the change in trend. A quadratic trend indicates a statistically significant but nonlinear change in prevalence over time. A long-term temporal change that includes a significant linear and quadratic trend demonstrates nonlinear variation (e.g., leveling off or change in direction) in addition to an overall increase or decrease over time. Cubic and higher-order trends were not assessed.

In reports that analyzed 2-year changes in health-related behaviors, prevalence estimates from 2019 and 2021 were compared by using t-tests for variables assessed with identically worded questions in both survey years. An exception was made for birth control use, where the wording specifically addressed sexual contact with opposite sex partners in 2021 but not in 2019. Prevalence estimates were considered statistically different if the t-test p value was <0.05. For 2-year changes assessed with absolute measures (i.e., prevalence difference), 95% CIs that did not cross zero were considered statistically significant. For relative measures (i.e., prevalence ratio), 95% CIs that did not cross 1.0 were considered statistically significant.

Data Availability and Dissemination

National and site-level YRBS data (1991–2021) are available in a combined data set from the YRBSS data and documentation website (https://www.cdc.gov/healthyyouth/data/yrbs/data.htm), as are additional resources, including data documentation and analysis guides. Data are available in both Access and ASCII formats, and SAS and SPSS programs are provided for converting the ASCII data into SAS and SPSS data sets. Variables are standardized to facilitate trend analyses and for combining data. YRBSS data also are available online via three web-based data dissemination tools: Youth Online, YRBS Analysis Tool, and YRBS Explorer. Youth Online allows point-and-click data analysis and creation of customized tables, graphs, maps, and fact sheets (https://nccd.cdc.gov/Youthonline/App/Default.aspx). Youth Online also performs statistical tests by health topic and filters and sorts data by race and ethnicity, sex, grade, and sexual orientation. The YRBS Analysis Tool allows real-time data analysis of YRBS data that generates frequencies, cross-tabulations, and stratified results (https://nccd.cdc.gov/YRBSSanalysis). YRBS Explorer is an application featuring options to view and compare national, state, and local data via tables and graphs (https://yrbs-explorer.services.cdc.gov). Data requests and other YRBSS-related questions can be sent to CDC by using the data request form (https://www.cdc.gov/healthyyouth/data/yrbs/contact.htm).