National Health and Nutrition

Examination Survey:

National Youth Fitness Survey

Plan, Operations,and

Analysis, 2012

Series 2, Number 163 April 2014
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Borrud L, Chiappa M, Burt V, et al. National Health and Nutrition Examination

Survey: National Youth Fitness Survey plan, operations, and analysis, 2012.

National Center for Health Statistics. Vital Health Stat 2(163). 2014.

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National Health and Nutrition Examination Survey. National Youth Fitness

Survey plan, operations, and analysis, 2012.

p. ; cm.— (Vital and health statistics. Series 2, Data evaluation and methods

research ; number 163) (DHHS publication ; no. (PHS) 2014–1363)

National Youth Fitness Survey plan, operations, and analysis, 2012

Includes bibliographical references.

ISBN 978-0-8406–0664-8 — ISBN 0-8406-0664-8

I. National Center for Health Statistics (U.S.), issuing body. II. Title: National

Youth Fitness Survey plan, operations, and analysis, 2012. III. Series: Vital and

health statistics. Series 2, Data evaluation and methods research ; no. 163. IV.

Series: DHHS publication ; no. (PHS) 2014–1363.

[DNLM: 1. National Youth Fitness Survey (U.S.) 2. Health Surveys—United

States. 3. Physical Fitness—United States. 4. Adolescent—United States. 5.

Child—United States. 6. Motor Activity—United States. 7. Statistics as

Topic—United States. W2 A N148vb no.163 2014]

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Series 2, Number 163

National Health and Nutrition
 

Examination Survey:
 

National Youth Fitness Survey Plan,
 

Operations,and Analysis, 2012
 

Data Evaluation and Methods Research

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Center for Health Statistics

Hyattsville, Maryland

April 2014

DHHS Publication No. 2014–1363
National Center for Health Statistics

Charles J. Rothwell, M.S., M.B.A., Director

Jennifer H. Madans, Ph.D., Associate Director for Science

Division of Health and Nutrition Examination Surveys

Kathryn S. Porter, M.D., M.S., Director
Contents
 

Abstract .................................................................................................. 1 
 

Introduction ............................................................................................... 1 
 

Planning and Sample Design ................................................................................. 2 
 

Children’s Physical Activity Feasibility Study.................................................................. 2 
 

Sample Design........................................................................................... 2 
 

Ethical, Privacy, and Confidentiality Considerations............................................................... 3 
 

Ethics Review Board...................................................................................... 3 
 

Informed Consent ........................................................................................ 3 
 

Field Operations ........................................................................................... 3 
 

Advance Arrangements .................................................................................... 4 
 

Staffing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 
 

Household Interview ...................................................................................... 4 
 

Mobile Examination Center Operations ......................................................................... 5 
 

Staffing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 
 

Safety Precautions ........................................................................................ 5 
 

Examinations ............................................................................................ 5 
 

Mobile Examination Center Interviews ....................................................................... 8 
 

Report of Findings and Remuneration .......................................................................... 9 
 

Data Release and Analytic Guidelines .......................................................................... 9 
 

Key Demographic Variables of Interest ....................................................................... 9 
 

Family Relationship...................................................................................... 10 
 

Geographic and Seasonal Considerations..................................................................... 11 
 

Effect of Nonresponse .................................................................................... 11 
 

Survey Sample Weights................................................................................... 12 
 

Variance Estimation...................................................................................... 13 
 

Statistical Reliability of Estimates .......................................................................... 13 
 

Estimation of Population Counts ........................................................................... 14 
 

Combining NNYFS and NHANES Data ..................................................................... 14 
 

Conclusions .............................................................................................. 14 
 

References ............................................................................................... 14 
 

Appendix. Glossary ........................................................................................ 16 
 

Text Figure

Diagram of mobile examination center, 2012 .................................................................... 6 
 

Text Tables

A. Children’s Physical Activity Feasibility Study, 2011 ........................................................ 2 
 

B. Examinations by sample domain, NNYFS 2012............................................................ 3 
 

C. Summary of forms used in the consent process, NNYFS 2012................................................ 3 
 

D. Sample person questionnaire, NNYFS 2012 ............................................................... 4 
 

iii
E. Family questionnaire, NNYFS 2012 .................................................................... 5 
 

F. Examination components, NNYFS 2012 ................................................................ 5 
 

G. Remuneration amounts, NNYFS 2012 .................................................................. 9 
 

H. Distribution of sample by age at screening, NNYFS 2012 .................................................. 10 
 

J. Distribution of sample by race and Hispanic origin, NNYFS 2012 ........................................... 10 
 

K. Unavailable values in data, NNYFS 2012................................................................ 12 
 

L. Appropriate use of weights, NNYFS 2012 ............................................................... 13 
 

iv
Background

In October 2008, the federal

government issued its first-ever

Physical Activity Guidelines for

Americans to provide science-based

guidance on the types and amounts of

physical activity that provide substantial

health benefits for Americans (1).

Guidelines for children and adolescents

recommend 60 minutes or more of

aerobic, muscle-strengthening, or

bone-strengthening physical activity

daily (1). While the number of children

in the United States who meet the

recommendations in the Physical

Activity Guidelines is unknown, the

percentage that is physically active in

the United States may be declining. No

recent national data exist on the fitness

levels of children and adolescents. The

National Health and Nutrition

Examination Survey’s (NHANES)

National Youth Fitness Survey (NNYFS)

was conducted in 2012 and collected

data on physical activity and fitness

levels for U.S. children and adolescents

aged 3–15 years.

Objectives

The objective of NNYFS was to

provide national-level estimates of the

physical activity and fitness levels of

children, based on interview and

physical examination data. Results from

the survey are intended to contribute to

the development of policies and

programs to improve youth fitness

nationally. The data also may be used

in the development of national

reference standards for measures of

fitness and physical activity.

Methods

The NNYFS survey design used the

design for NHANES, which is a

multistage probability sample of the

civilian noninstitutionalized resident

population of the United States. NNYFS

consisted of a household interview and

a physical activity and fitness

examination in a mobile examination

center. A total of 1,640 children and

adolescents aged 3–15 were

interviewed, and 1,576 were examined.

Keywords: physical activity in

children • mobile examination center

National Health and Nutrition

Examination Survey: National

Youth Fitness Survey Plan,

Operations, and Analysis, 2012

by Lori Borrud, Ph.D., Michele M. Chiappa, B.A., Vicki L. Burt,

Sc.M., R.N., Jaime Gahche, M.P.H., George Zipf, M.S., National

Center for Health Statistics; Sylvia M. Dohrmann, M.S., Westat; and

Clifford L. Johnson, M.S.P.H., National Center for Health Statistics

Introduction
 

The first-ever Physical Activity

Guidelines for Americans were issued

by the federal government in October

2008. The guidelines provided

science-based guidance on the types and

amounts of physical activity that provide

substantial health benefits for Americans

(1). Sixty minutes or more of physical

activity daily is recommended for

children and adolescents. Activities

include moderate-intensity or

vigorous-intensity aerobic activity (e.g.,

running, karate, or bicycle riding);

muscle-strengthening activity (e.g., tree

climbing, swinging, or gymnastics); or

bone-strengthening activity (e.g.,

hopping, skipping, or jumping) (1).

One of every three children in

the United States is overweight or obese

(2). This may increase children’s risk of

developing diabetes, heart disease, and

cancer over the course of their lives.

Regular physical activity in children and

adolescents promotes health and fitness

and may help prevent obesity (1).

Estimates of the number of U.S.

children who meet the physical activity

guidelines are limited. Data from the

2001–2009 Health Behavior in

School-aged Children quadrennial

surveys, nationally representative

samples of U.S. children in the 6th

through 10th grades, identified

significant increases in the number of

days per year with at least 60 minutes

of physical activity (3). However, data

from the Centers for Disease Control

and Prevention’s (CDC) Youth Risk

Behavior Surveillance System indicate

that the percentage of physically active

U.S. children in the 9th through 12th

grades is declining (4).

The National Health and Nutrition

Examination Survey (NHANES) has

administered tests of physical activity

and fitness that included children and

adolescents. A physical activity monitor

was fielded for participants aged 6 years

and over in 2003–2006 and was

implemented again in 2011. In

1999–2004, cardiovascular fitness was

reported as estimated maximal oxygen

uptake (VO2 max) from a treadmill test

for participants aged 12–49. A grip

strength component was implemented in

2011 for participants aged 6 and over.

However, the inclusion of additional

tests of physical activity and the

expansion on tests previously done in

NHANES for children as young as age

3 years would provide further

information to evaluate changes in the

health of children and adolescents over

time. The NHANES National Youth

Fitness Survey (NNYFS) was the first

national survey of physical activity and

fitness in children and adolescents and

was conducted jointly with NHANES in

2012. For information on NHANES,

visit http://www.cdc.gov/nchs/

nhanes.htm.

Page 1
Page 2 [ Series 2, No. 163

Planning and Sample

Design

In 2011, CDC’s National Center for

Health Statistics (NCHS) received

Affordable Care Act funds from the

U.S. Department of Health and Human

Services (HHS) to plan, implement, and

conduct NNYFS. No national study of

the physical fitness of U.S. children had

been conducted since the mid-1980s.

The HHS Secretary had dedicated funds

to obesity prevention and fitness—one

of four critical priorities for the

department (1). The obesity prevention

and fitness priority aims to ‘‘advance

activities to improve nutrition and

increase physical activity to promote

healthy lifestyles and reduce obesityrelated conditions and costs’’ (5).

The NNYFS sample was designed

so that data from components common

to NNYFS and NHANES in 2012, such

as the body measures, physical activity

monitor, muscle strength (grip strength),

physical activity questionnaire, and

dietary recall could be analyzed

together. NNYFS included additional

tests of cardiovascular capacity,

performance endurance, core strength,

upper and lower body strength, and

gross motor skills, as well as questions

on health characteristics and drug,

alcohol, and tobacco use. The

NHANES–NNYFS project involved

identifying which activities could be

conducted in both surveys to provide a

larger sample size for analysis,

developing an innovative sample design

that incorporated both surveys, training

interviewers to administer questionnaires

for both studies simultaneously, and

refurbishing NHANES trailers for a

new use.

The planning and implementation of

NNYFS data collection took about 8

health examiners. Challenges included

incorporating 10 NNYFS components

into the limited space of a single trailer

and a tight survey schedule. Screening,

interviewing, and examining for NNYFS

were accelerated because the mobile

examination centers (MECs) were

operating at each survey location

between 21⁄2 and 3 weeks, compared

with 5 to 6 weeks for NHANES.

Children’s Physical

Activity Feasibility Study

Planning for a feasibility study of

NNYFS components began in early

spring 2011, with input from recognized

experts in physical activity and

kinesiology from within the federal

government and academia and from a

review of the scientific literature. Six

tests were selected or developed by staff

from NCHS’ Division of Health and

Nutrition Examination Surveys

(DHANES) that would provide

nationally representative data on fitness

and physical activity while still being

conducted in the limited space available

in a single trailer. To provide the most

accurate evaluation of the feasibility of

these tests in a confined mobile

environment, DHANES staff conducted

the Children’s Physical Activity

Feasibility Study (CPAFS) in a

refurbished NHANES trailer in 2011.

One hundred sixty-nine children aged

3–15 were recruited and participated in

the study. Necessary consent was

obtained prior to participation. CPAFS

included one test to measure physical

activity, five tests of endurance and

muscle strength, and one measurement

of height and weight (Table A).

Sample Design

NNYFS was conducted concurrently

with NHANES in 2012, the second year

of the NHANES 2011–2012 survey

cycle, with survey participants selected

from an independent sample of dwelling

units (DUs) within the segments

selected for that year of NHANES. The

minimum measure of size for the 2012

segments was increased above the rates

established for the NHANES 2011–2014

sample to ensure sufficient DUs for both

surveys in the selected segments.

NHANES and NNYFS used four

stages of sampling. First, primary

sampling units (PSUs) (mostly counties)

were selected to meet the analytic goals

for NHANES. Second, segments large

enough to ensure there were enough

DUs for both surveys were selected

within PSUs. The third and fourth stages

involved selecting DUs or households

specific to NNYFS and individuals

within those households, so that no

households had both NHANES and

NNYFS participants. The NNYFS

design differed from that of NHANES

in that participants were selected based

on sex and age only (income and race

and Hispanic origin were not selection

criteria).

The goal for NNYFS was to

interview and examine approximately

1,500 children and adolescents aged

3–15 with approximately equal sample

sizes for each single year of age. In

addition, six analytic domains (males

3–5, females 3–5, males 6–11, females

6–11, males 12–15, and females 12–15)

were defined for the survey. The

targeted number of examinations and

actual number of achieved interviews

Table A. Children’s Physical Activity Feasibility Study, 2011

months, which included the refurbishing Target

Test age (years) of three NHANES trailers, preparing

protocols for the physical activity and

fitness tests, planning and conducting a

feasibility study of the tests, purchasing

equipment to outfit the refurbished

trailers, developing a physical activity

questionnaire, and hiring and training

Physical activity monitor—accelerometer............................ 3–5

Endurance performance—treadmill ............................... 6–11

Lower body muscle strength (sitting and prone positions, extension, and flexion) . . . 6–15

Modified pull-up (upper body muscle strength) ........................ 5–15

Plank exercise (core strength and muscular endurance) .................. 3–15

Gross motor skills (balance and coordination) ........................ 3–5
Series 2, No. 163 [ Page 3

Table B. Examinations by sample domain, NNYFS 2012 the parent or guardian for the selected

Target number Number Number of

of examinations of interviews examinations

Sex and age (years) domain (percent) (percent) (percent)

Males, 3–5 ................. 173 (11.5) 187 (11.4) 179 (11.3)

Males, 6–11. . . . . . . . . . . . . . . . . 346 (23.1) 377 (23.0) 358 (22.7)

Males, 12–15................ 231 (15.4) 259 (15.8) 250 (15.9)

Females, 3–5. . . . . . . . . . . . . . . . 173 (11.5) 181 (11.0) 173 (11.0)

Females, 6–11 ............... 346 (23.1) 385 (23.5) 374 (23.7)

Females, 12–15 .............. 231 (15.4) 251 (15.3) 242 (15.4)

Total, 3–15 ................. 1,500 (100.0) 1,640 (100.0) 1,576 (100.0)

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.

and examinations, by the six age-sex

analytic domains, are shown in Table B.

A total of 2,065 children and

adolescents aged 3–15 were selected to

participate in the survey. Of these, 1,640

(79.4%) were interviewed and 1,576

(76.3%) were examined.

Ethical, Privacy, and

Confidentiality

Considerations

The NNYFS protocol was

developed to comply with the HHS

Policy for Protection of Human

Research Subjects (45 CFR part 46)

(http://www.hhs.gov/ohrp/human

subjects/guidance/45cfr46.html) and was

approved by the NCHS Research Ethics

Review Board (ERB). All data collected

were protected by three federal laws: the

Privacy Act of 1974 (5 U.S.C. 552a),

section 308(d) of the Public Health

Service Act (42 U.S.C. 242m), and the

Confidential Information Protection and

Statistical Efficiency Act (PL107–347).

The Public Health Service Act stipulates

that ‘‘... no information may be used for

any purpose other than the purpose for

which it was supplied unless... [a]

person consented to its use for such

other purpose, and further that it cannot

be released or published in a form that

the particular ... person supplying the

information or described in it is

identifiable unless such ... person has

consented ... .’’ These strict prohibitions,

forbidding even unintentional,

unauthorized disclosures, guided the

behavior of all NCHS and contractor

staff. Any violation of these laws could

result in fines or imprisonment.

Ethics Review Board

The NCHS Research ERB protected

the rights and welfare of NNYFS

participants. In accordance with Federal

Regulation 45 CFR 46.111, the NCHS

Research ERB reviewed and approved

NNYFS protocols and any changes.

Informed Consent

The informed consent process for

NNYFS followed the procedures

established for NHANES. If the parent

or guardian for one or more sample

persons identified through the screening

process was available, the interviewer

continued with the home interview. If

sample person was not available, the

interviewer made an appointment to

return to administer the sample person

and family questionnaires. Documented

signed consent was obtained from each

sample person’s parent or guardian

(required to be age 18 or over) prior to

the household interview and the

appointment at the NNYFS MEC.

Sample persons aged 7–15 provided

additional signed assent to participate in

the examination portion of NNYFS.

Participants were provided a copy of

their signed parent or guardian consent

and a consent brochure, which included

information on participation. The

consent brochure is available on the

NNYFS website at http://www.cdc.gov/

nchs/nnyfs/brochures.htm.

In addition, participants aged 12–15

answered the questions in the Physical

Activity section of the sample person

questionnaire themselves and signed a

consent form for that section. The

appointment for the MEC examination

was made after the home interview was

completed. If participants were not

available at the time of the household

interview, they were able to consent to

the physical activity questions at the

MEC before their examination. Table C

provides a summary of the forms used

to complete the consent process for

NNYFS.

Field Operations

NNYFS was operationally designed

to achieve high response rates—a

minimum of 1,500 children—and

high-quality data. Field operations

included advance arrangement activities,

staffing, training, and interviewing.

Table C. Summary of forms used in the consent process, NNYFS 2012

Home interview Parental Parental

consent for Home interview permission to consent or

Participant participants assent for physical audiotape household assent permission Child assent

age (years) aged 3–15 activity questions interview for examination for examination

3–5. . . . . . . . . . . . . . . . . . . . . . . . . . . Signed by parent . . . Signed by parent Signed by parent . . .

6–11 .......................... Signed by parent . . . Signed by parent Signed by parent Signed by participant

(ages 7–11)

12–15 ......................... Signed by parent Signed by participant Signed by parent Signed by parent Signed by participant

. . . Category not applicable.
 

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.
 
Page 4 [ Series 2, No. 163

Advance Arrangements

All DUs selected for NNYFS were

sent an advance letter approximately 3

weeks before interviewing began that

introduced NNYFS, provided contact

information for NCHS, and explained

that an interviewer would be making a

home visit. All other advance

arrangement activities for NNYFS, such

as finding locations for the trailers and

notifying local community leaders, were

performed in conjunction with

NHANES. Details of advance

arrangement activities are available in

‘‘The National Health and Nutrition

Examination Survey: Plan and

Operations, 1999–2010’’ (6).

Staffing

The field staff included the field

office and interviewers. The field office

consisted of a study manager, field

manager, office manager, and two

assistant office managers at each survey

location. The field office supported both

NHANES and NNYFS with regard to

household interviewing and examining

data collection. The interviewers also

supported both surveys and were

proficient in contacting residents of

sampled households. Interviewers

administered the screener questionnaire,

obtained all consents, and administered

all household interview questions.

Household Interview

Screener

The NNYFS screener module

consisted of a set of questions to

determine if any child in the household

was eligible to participate in the survey.

The interviewer verified the household

address and asked questions regarding

the number of people living in the

household, the names of household

members, and their age and sex. This

was a very brief interview and was

conducted at the door. If an eligible

child was identified, the interviewer

proceeded with the household interview.

Circumstances preventing an interviewer

from completing a screener

questionnaire included:

+	 A vacant unit or residence

+	 An address not qualifying as a DU

+	 A nonexistent sampled DU

+	 A household resident refusing to

participate

Sample person and family

questionnaires

The household interview included

administration of the sample person

questionnaire and a short family

questionnaire. The sample person

questionnaire collected information on

the participant’s demographic,

socioeconomic, dietary, and healthrelated history (Table D). It was

administered to all eligible participants

or their proxies. Participants aged 12–15

were required to answer the questions in

the Physical Activity section themselves.

If they were not at home at the time of

the household interview, they were

asked the physical activity questions at

the MEC.

The family questionnaire (Table E)

was completed for every family unit

(e.g., a married couple, a couple living

as married, and others) sampled within

the household. The U.S. Census

Bureau’s Current Population Survey

family definitions were used (7). The

family questionnaire included questions

on the country of birth and level of

education of the nonsampled head of the

household and on family income. All

NNYFS questionnaires are available on

the NNYFS website at http://www.cdc.

gov/nchs/nnyfs/questionnaires.htm.

At the conclusion of the household

interview, the interviewer scheduled an

appointment for the participant to be

examined in the NNYFS MEC. A

reminder letter was mailed to the

participant 1 week before the scheduled

examination. The letter included the date

and time of the examination

appointment, transportation information,

directions to the MEC, instructions on

what to wear and bring, and the

remuneration amount. The letter also

emphasized the requirement that all

children be accompanied by an

individual aged 18 or over. Forty-eight

hours before an examination

Table D. Sample person questionnaire, NNYFS 2012

Questionnaire section Description

Acculturation (ACQ).................. Primary language(s) used at home, and mother’s and father’s countries of origin

Demographics (DMQ) ................ Level of education, race and Hispanic origin, and country of origin

Diabetes (DIQ)..................... History of diabetes and prediabetes; now taking insulin or diabetic pills

Dietary supplements and prescription

medication (DSQ) .................. Use of supplements, nonprescription antacids, and prescription medications

Diet behavior and nutrition (DBQ) ......... School breakfast and lunch program utilization, number of meals eaten away from home, use of ready-to-eat foods,

frozen meals, and frozen pizzas

Early childhood (ECQ) ................ Whether mother smoked while pregnant, weight at birth, and history of overweight

Health insurance (HIQ)................ Health care coverage and type of insurance

Hospital utilization and access to care (HUQ) . . General health status, place goes to most often when sick

Medical conditions (MCQ) .............. History of asthma, and use of medication for asthma, trouble seeing, and menstruation

Physical activity (PAQ) ................ Vigorous- or moderate-intensity activities and sports and frequency and length of time spent in those activities,

walking and bicycling for travel, sitting, television viewing, and computer and video game use (additional questions

were asked of participants aged 12–15)

Physical functioning (PFQ) ............. Physical impairments, use of special equipment, participation in special education, or use of early intervention

services

Respiratory health and disease (RHQ) ...... Wheezing and limitation of activity due to wheezing or whistling

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.
Series 2, No. 163 [ Page 5

Table E. Family questionnaire, NNYFS 2012	 protocols, and maintaining supply

inventory. Questionnaire section	 Description

Demographic background or occupation (DMQ–FAM) . . .	 Level of education, race and Hispanic origin, and

country of origin of nonsample participant head Safety Precautions

of household

Income (INQ)............................ Family and total household income All children were accompanied to

the MEC by an individual aged 18 or

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.

over. The nurse practitioners

appointment, the field office staff called

the participant’s home as a reminder of

the upcoming examination. Instructions

related to the examination were

reviewed with the participant or the

parent or guardian. If the participant did

not have a phone, the interviewer visited

the home to provide an in-person

reminder.

Mobile Examination

Center Operations

The NNYFS MEC consisted of a

single trailer that traveled with the four

NHANES MEC trailers, but it was not

physically connected to any of the

NHANES trailers at survey sites. All

locations for the NHANES trailers were

chosen to accommodate the additional

stand-alone NNYFS trailer. The NNYFS

trailer was designed to accommodate

physical activity and fitness

examinations and interviews, and it

consisted of two large examination

rooms and one interview room (Figure).

Table F. Examination components, NNYFS 2012

The MEC was a nonsmoking facility.

Each MEC was equipped with safety

equipment and supplies.

Staffing

NNYFS employed one examination

team that spent 21⁄2 to 3 weeks at each

survey location. The MEC staff was

selected based on their previous

experience with children, academic

training, knowledge, skills, and abilities.

The staff included two nurse

practitioners and two examiners with

degrees in exercise physiology and

nutrition or kinesiology. Two of the staff

members were bilingual to address

language needs of Spanish-speaking

survey participants. Training for the

MEC staff was rigorous and included

learning the objectives and goals of each

examination component, demonstrating

protocols and procedures, live practicing

of protocols and procedures with the

contractor’s home office staff and paid

volunteers, learning the quality control

and quality assurance procedures,

maintaining and calibrating equipment,

learning MEC opening and closing

administered the ‘‘Nurse Review,’’

which asked the parent or guardian

questions regarding physical disabilities,

injuries, surgeries, or medications that

would exclude their child from

participating in a component. ‘‘The

Nurse Review Program Manual’’ can be

found at http://www.cdc.gov/nchs/data/

nnyfs/Nurse_Review.pdf. MEC staff

were present during all examination

components, which were stopped

immediately when any participant

complained of discomfort or pain. All

components also were stopped

immediately if the participant was not

able to follow instructions sufficiently.

Examinations

The NNYFS examination

components had either been

administered in NHANES or tested in

CPAFS and included standardized

measurements of anthropometry, core

strength, upper and lower body muscle

strength, cardiovascular fitness, and

gross motor development. The NNYFS

examination components are listed in

Table F.

Component Target age (years) Source Report of findings

Body measures ........................ 3–15 NHANES Yes (height, weight, and body mass index)

Core muscle strength (plank)................ 3–15 CPAFS No

Gross motor skills ...................... 3–5 CPAFS No

Lower body muscle strength (extension at the knee

in sitting position) ...................... 6–15 CPAFS No

Upper body muscle strength (grip strength)....... 6–15 NHANES Yes (ages 7–15 only)

Upper body muscle strength (modified pull-up)..... 5–15 CPAFS Yes

Physical activity monitor (accelerometer) ........ 3–15 NHANES and CPAFS No

Aerobic fitness (maximal and submaximal exercise

test) .............................. 6–15 NHANES and CPAFS Yes (ages 12–15 only)

Computer-assisted self-interview ............. 12–15 NHANES No

Dietary recall interview ................... 3–15 NHANES No

Physical activity questionnaire (if not administered in

the household interview).................. 12–15 NHANES No

NOTES: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey. NHANES is National Health and Nutrition Examination Survey. CPAFS is Children’s Physical

Activity Feasibility Study.
Examination room 2 Examination room 1 

Interview 

room 

Bathroom 

Reception 

Wheelchair 

lift 

Entry 

Page 6 [ Series 2, No. 163

SOUR CE: CD C/NCH S, Nat io n al H eal t h an d Nu t r i t io n Ex am in at io n Su r v ey, Nat io n al Yo u t h Fit n ess Su r v ey, 2012. 

Figure. Diagram of mobile examination center, 2012
 
Series 2, No. 163 [ Page 7

All participants were asked to dress

in comfortable, loose-fitting, shortsleeved or sleeveless shirts and shorts.

Participants wore running or tennis

shoes and socks. Shorts and shoes were

available for children who did not come

appropriately dressed for the

examination.

The controlled environment of the

MEC allowed physical measurements to

be conducted under identical conditions

at each survey site. Eligibility for

examination components was

determined by the participant’s age and

any exclusions identified during the

Nurse Review. The following sections

describe the MEC examination and

interview components. More detailed

information on the components is

available on the NNYFS website

(http://www.cdc.gov/nchs/nnyfs.htm).

Body measures

All participants aged 3–15 were

eligible for the body measures

component. Measurements included

height and weight; lengths of the entire

arm and upper arm; midarm, midcalf,

and waist circumferences; and

subscapular, triceps, and calf skinfold

thickness. The NNYFS body measures

component included the same

standardized methods that have been

employed in NHANES since the first

survey was conducted in 1960–1962.

Height, weight, and body mass index

(calculated as weight in kilograms

divided by height in meters squared)

were included in the ‘‘Report of

Findings.’’ More information is available

in the ‘‘Body Measures Procedures

Manual’’ at http://www.cdc.gov/nchs/

data/nnyfs/Body_Measures.pdf.

Core muscle strength—plank

Participants aged 3–15 were eligible

for the plank or core muscle strength

component. The plank exercise assessed

muscular endurance and core strength

around the trunk and pelvis. The

component used the front plank

position, in which the participant lies

face down on a mat, resting on the

forearms with hands clenched or palms

flat on the floor. The participant then

pushes off the floor, rising up onto toes,

and maintains the position as long as

possible while keeping the back straight

without the stomach dropping or hips

rising up. No results were reported to

the participants. These data will provide

the first nationally representative data on

core strength for children and

adolescents. For more information, see

the ‘‘Plank Exercise Procedures

Manual’’ (http://www.cdc.gov/nchs/data/

nnyfs/Plank.pdf).

Gross motor skills

Participants aged 3–5 were eligible

for the gross motor skills component.

Gross motor skills involve the large

muscles of the body that enable major

body movement such as walking,

maintaining balance, coordination,

jumping, and reaching. Children with

better-developed motor skills may find it

easier to be active and engage in more

physical activity than those with

less-developed motor skills.

The Test of Gross Motor

Development (TGMD–2) (8) was used

for this component. TGMD–2 has been

used by researchers in the United States

and internationally and is a normreferenced measure of common gross

motor skills that develop early in life.

TGMD–2 consists of two subtests with

six skills for each subtest: locomotor

(running, galloping, hopping, leaping,

horizontal jumping, and sliding); and

object control (striking a stationary ball,

stationary dribbling, kicking, catching,

overhand throwing, and underhanded

rolling). The developer of TGMD–2

provided consultation on the component.

No results were reported to the

participant. The gross motor skills

component will provide the first

nationally representative data on

locomotion and object-control skills for

young children. For more information,

see the gross motor skills procedures

manual (http://www.cdc.gov/nchs/data/

nnyfs/TGMD.pdf).

Lower body muscle strength

Participants aged 6–15 were eligible

for the lower body muscle strength

(LBMS) component. A hand-held

dynamometer (HHD) was used to assess

LBMS (quadriceps strength) through

measurement of maximum isometric

knee extension force in the sitting

position. The LBMS chair was

specifically developed and built for

isometric testing. The participant’s hips,

thighs, and upper body were secured on

the test chair with straps. Isometric

resistance was supplied using a belt

passing through a strap on the HHD and

around the chair. Participants were asked

to push their legs as hard as possible

against the HHD in three tests of each

leg, alternating legs. A professor of

kinesiology, physiology, and

neurobiology provided consultation for

the component. No results were reported

to the participant. The LBMS

component will provide the first

nationally representative LBMS data for

children and adolescents. For more

information, see the lower body muscle

strength procedures manual at

http://www.cdc.gov/nchs/data/nnyfs/

Lower_Body_Muscle_Strength.pdf.

Upper body muscle strength—

grip strength

Participants aged 6–15 were eligible

for the muscle strength or grip strength

component. NHANES implemented the

isometric grip strength test in 2011 to

provide data for comparison with data

on upper body muscle strength from

National Health Examination Survey

cycles II and III, 1963–1970, which

included a grip test for children aged

6–11 and 12–17, respectively. The

NHANES grip strength test was

developed in collaboration with the

National Cancer Institute.

NNYFS used the same protocol for

the grip strength test that was used in

NHANES. Participants were asked to

squeeze a dynamometer adjusted for

their hand size as hard as possible three

times in each hand, alternating hands. A

derived variable (in pounds of force)

created from the sum of the highest

reading from each hand was included in

the Report of Findings. The

classification of strength included in the

Report of Findings for ages 7–14 was

based on reference data from the

Canada Fitness Survey Longitudinal

Study (9) and, for age 15, from the

Canadian Physical Activity, Fitness &

Lifestyle Approach (10).
Page 8 [ Series 2, No. 163

Results were not reported to participants

aged 6 years. For more information, see

the grip strength procedures manual at

http://www.cdc.gov/nchs/data/nnyfs/

Handgrip_Muscle_Strength.pdf.

Upper body muscle strength—

modified pull-up

Participants aged 5–15 were eligible

for the modified pull-up component. The

modified pull-up measures upper body

muscle (back, shoulder, forearm, and

arm) strength. The component used a

specially constructed horizontal bar

positioned at a height that allowed

participants to clasp the bar with an

overhand grasp when lying flat on their

back. Participants raise their body by

flexing the arm until the chest touches a

strap hanging down from the horizontal

bar, then lower back to the starting

position while keeping the body straight

with the hips up and only the heels

touching. The procedure was repeated as

many times as possible. The number of

correctly completed pull-ups was

compared with the number of age- and

sex-specific Healthy Fitness Zone

pull-ups in FitnessGram and provided to

participants in their Report of Findings.

FitnessGram is a battery of healthrelated fitness items scored using

criterion-referenced standards (11). The

modified pull-up component will

provide the first nationally

representative data against which

schools can compare results from their

districts. For more information, see the

modified pull-up procedures manual at

http://www.cdc.gov/nchs/data/nnyfs/

Modified_Pullup.pdf.

Physical activity monitor—

accelerometer

All participants aged 3–15 were

eligible for the physical activity monitor

(PAM) component, including children

who were wheelchair-bound. At the

MEC, monitors were placed on the wrist

of the participant’s nondominant arm.

Participants were asked to wear the

monitors 24 hours a day for 7 full days,

with measurements beginning the day

after their examination. The monitors

were water-resistant and could be worn

while swimming, showering, or bathing.

While a PAM component for measuring

physical activity was fielded in

NHANES in 2011 for ages 6 and over,

to be consistent with the NNYFS

component, eligibility was changed in

2011 to include ages 3–5. The same

accelerometer used in NHANES was

used in NNYFS. After the monitors

were returned in provided postage-paid

envelopes, remuneration checks for $40

were mailed to the survey participants.

No results were reported to participants

from the PAM component. Currently,

there are no reporting standards. Data

collected in the 2012 NHANES and

NNYFS will provide the first nationally

representative data on measured physical

activity levels for children aged 3–5. For

more information, see the ‘‘Physical

Activity Monitor Procedures Manual’’ at

http://www.cdc.gov/nchs/data/nnyfs/

Physical_Activity_Monitor.pdf.

Aerobic fitness—maximal and

submaximal exercise test

Participants aged 6–15 were eligible

for the treadmill component. Two

treadmill protocols were used in

NNYFS. The protocol developed for

ages 6–11 measured endurance

performance based on duration on the

treadmill, with increasing speed and

incline. The protocol had been approved

by the NCHS Research ERB and tested

successfully in CPAFS. The treadmill

protocol for ages 12–15 was a

submaximal exercise test that had been

conducted previously in NHANES

1999–2006 to measure aerobic capacity

(http://www.cdc.gov/nchs/nhanes/

nhanes2003-2004/CVX_C.htm). A

treadmill designed specifically for

exercise testing was used for both

protocols. Four leads with electrodes

were attached to the participant’s chest

to measure heart rate.

Participants were given time to

practice before test initiation in order to

familiarize themselves with treadmill

walking and a cool-down period at the

end of the test. Two examiners were

present during the treadmill test: one

monitoring the heart rate data and

treadmill and the other encouraging and

monitoring the participant. As the test

proceeded, the examiner continually

assessed the participant by direct

observation and by reviewing heart rate

readings. The treadmill component

included two levels of priority criteria

that warranted an emergency stop of the

protocol. Stopping criteria included

distress observed by the health examiner

or reported by the participant, a request

by the participant to stop, and

equipment failure. A pediatric

cardiologist provided consultation for

the treadmill component. Results were

not reported to participants aged 6–11

because there are no established

standards against which to evaluate

endurance performance in children. The

Report of Findings for participants aged

12–15 provided incline, speed, duration,

and general fitness classification,

relative to the same sex and age group,

based on estimated maximum work

capacity (VO2 max) in FitnessGram

(11). For more information, see the

treadmill procedures manual at

http://www.cdc.gov/nchs/data/nnyfs/

Treadmill.pdf.

Mobile Examination

Center Interviews

Physical activity questionnaire

The physical activity questionnaire

was administered to participants aged

12–15 in the MEC only if the

participant was unavailable to answer

questions during the household

interview; see Table D for more

information on the physical activity

questionnaire.

Dietary recall

All participants aged 3–15 were

eligible for the dietary recall interview.

The dietary interview for NNYFS was

the same interviewer-administered,

computer-assisted, 24-hour dietary recall

administered in NHANES. NNYFS

included only the in-person interview in

the MEC; unlike NHANES, the

second-day telephone interview was not

conducted. The objective of the dietary

interview component was to obtain

detailed dietary intake information from

NNYFS participants on the types and

amounts of foods and beverages

(including all types of water) consumed
Series 2, No. 163 [ Page 9

during the 24-hour period prior to the Table G. Remuneration amounts, NNYFS 2012

interview (midnight to midnight). The

dietary interview also included questions

on dietary supplements (e.g., vitamins,

minerals, herbals, and nonprescription

antacids) consumed during the 24-hour

period. The dietary interview component

(called ‘‘What We Eat in America’’) is

conducted as a partnership between the

U.S. Department of Agriculture and

HHS. The questions were included in

NNYFS to be used as covariates in the

analysis of the physical activity and

fitness data.

Computer-assisted selfinterview

A short computer-assisted

self-interview (CASI) questionnaire was

completed by participants aged 12–15 in

the interview room on a touch-screen

monitor. The questionnaire did not have

an audio component. A small number of

questions from the NHANES Tobacco

Use, Alcohol Use, and Drug Use

questionnaire sections were included in

the self-interview. There were no proxy

respondents for CASI, and MEC staff

were not present. The questions were

included in NNYFS to be used as

covariates in the analysis of the physical

activity and fitness data.

Report of Findings

and Remuneration

All NNYFS Reports of Findings

were provided in the MEC at the end of

the examination appointment; none were

mailed. The Report of Findings was

given to participants aged 12 and over

and to the parent or guardian of younger

participants. The Report of Findings was

provided for components for which

standards were available and against

which results could be measured.

Several of the NNYFS physical activity

and fitness components had not been

conducted on a national level previously,

and therefore standards were not

available. If the participant was

accompanied by an adult other than the

parent or guardian, the child received

the Report of Findings in a folder

Circumstance Payment (in dollars)

Participation by children aged 12–15 years ................ 60–75

Participation by children under age 12 years................ 40–55

Return of the physical activity monitor.................... 40

Parent incentive ................................. 20

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.

labeled ‘‘For the attention of [the child’s

parent or guardian].’’

The average participant burden for

NNYFS was estimated at 2 hours: 30

minutes for the household interview and

90 minutes for the trailer examination.

However, because the actual time

burden depended on the participant’s

age, the remuneration amount also

varied by participant age. For example,

the examination time was least for

participants aged 3–5 and greatest for

those aged 12–15. Therefore,

participants aged 12–15 received higher

remuneration. See Table G for

remuneration amounts. The

remuneration was provided at the end of

the participant’s examination

appointment. The parent or guardian of

each participant also was given a

monetary incentive, a transportation

allowance (not to exceed $70.00 for

driving the participant to the

examination appointment), and childcare

expenses (up to $5.25 per hour for up to

6 hours), if necessary.

Data Release and

Analytic Guidelines

The NNYFS data are publicly

available and may be downloaded freely

from the NNYFS data page (http://

wwwn.cdc.gov/nchs/nhanes/search/

nnyfs12.aspx). Data files are released as

SAS transport files in .XPT format, but

they may be used with any package that

supports this file format, such as

SUDAAN, SPSS, or STATA. Note that

NNYFS is a complex probability

sample, and proper analysis of the data

requires statistical software that

specifically incorporates sample design

complications such as weighting and

clustering. Data that cannot be released

publicly are available through NCHS’

Research Data Center [(RDC)

http://www.cdc.gov/rdc/].

Prior to beginning any analysis, the

reader is strongly encouraged to read all

relevant documentation to understand

how data were collected and released,

and to understand sample sizes, coding

of missing values, skip patterns for

questionnaire sections, and other

relevant information. Each data file

name on the NNYFS data page contains

a ‘‘Doc File’’ link, which allows the

reader to view the documentation,

including the codebook with the

frequency distribution for each item in a

particular data file.

The recommended approach for the

analysis of NNYFS data is design-based

analysis. Design-based analytic

procedures explicitly consider features

of the survey design, such as differential

selection probabilities and geographic

clustering. Important resources for all

NHANES analyses, including NNYFS,

are the ‘‘National Health and Nutrition

Examination Survey: Analytic

Guidelines, 1999–2010’’ (12) and the

NHANES Web tutorials (13), which are

designed to assist NHANES users in

understanding and analyzing the data.

The most important consideration in

analyzing NNYFS data is the survey

design. Survey sample weights should

be used, and the complex survey design

must be taken into account for variance

estimation.

Key Demographic

Variables of Interest

Two age variables are released with

the public data files. RIDAGEYR, (age

at screening) records the best age in

years of the sample child at time of the

household screening, and RIDEXAGY

(age at examination) records the best

age in years at date of examination. Age

at screening was used to determine
Page 10 [ Series 2, No. 163

eligibility for an examination

component. Each respondent’s actual or

imputed date of birth was used to

calculate RIDAGEYR. NCHS used a

Table H. Distribution of sample by age at screening, NNYFS 2012

Age (years) at screening

(RIDAGEYR)

Number of

interviewed children

(RIDSTATR = 1 or 2)

Number of

examined children

(RIDSTATR = 2)

procedure to impute age at screening,

when the date of birth was missing or

refused but the respondent’s age in

years was provided:

+	 If month of birth was missing or not

given, it was imputed as 7.

+	 If day of birth was missing or not

given, it was imputed as 1.

+	 If the year of birth was missing or not

given, it was imputed as the year of

the screening interview minus the age

in years provided by the respondent

during the screening interview.

Read the ‘‘Demographic Variables and

Sample Weights’’ data documentation on

the NHANES website (http://www.cdc.

gov/nchs/nnyfs/Y_DEMO.htm) for

further detail on these age variables.

Defining age group categories

Age groups used in an analysis

should be determined by what is most

appropriate for the specific analysis, in

conjunction with established statistical

reliability criteria. Although single year

of age is provided in the publicly

released data, the sample sizes for such

a detailed age classification are too

small for meaningful analysis, so some

form of age grouping is required. When

possible, age grouping should be

consistent with the survey design

groupings: 3–5, 6–11, and 12–15 years.

Table H provides the interview and

examination sample sizes for these age

groupings. The NNYFS questionnaire

items and examinations were

administered to participants in age

ranges that correspond to these sample

design age groups.

Race and Hispanic origin

Race and Hispanic origin was

determined based on self-identification

(or parent identification) by respondents

with five categories reported in the

released data. The race and Hispanic

origin variable, RIDRETH1, was

derived from responses to the survey

questions on race and Hispanic origin.

3–5 ............................... 
 

6–11 ............................... 
 

12–15 .............................. 
 

Total ............................... 
 

n (unweighted percent)

368 (22.4) 352 (22.3)

762 (46.5) 732 (46.5)

510 (31.1) 492 (31.2)

1,640 (100.0) 1,576 (100.0)

NOTES: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey. RIDSTATR is the interview

and examination status of the sample person.

Respondents who self-identified as

Mexican American or other Hispanic

(i.e., RIDRETH1 = 1 or 2, respectively)

were categorized as such, regardless of

their other race identities. Categories for

non-Hispanic persons who self-identified

as white or black (i.e., RIDRETH1 = 3

or 4) include only those who reported a

single race. All non-Hispanic persons

reporting multiple races were

categorized in the ‘‘Other race’’

category. Table J shows the NNYFS

sample distribution by RIDRETH1.

The RIDRETH1 variable is

consistent with the race and Hispanic

origin variable available for NHANES

survey cycles in that the MexicanAmerican and other Hispanic categories

may include persons of multiple races,

and the non-Hispanic white and black

categories include only those reporting a

single race.

Because NNYFS (unlike NHANES)

does not contain oversamples of any

race or Hispanic origin subgroup,

analyses of subgroups (e.g., the

above-mentioned age categories) within

categories of RIDRETH1 may be based

on small sample sizes. Therefore,

analyses by race and Hispanic origin

previously performed using NHANES

data may not be possible.

Family Relationship

NNYFS was not designed to

produce estimates at the household or

family level, only at the person level. As

such, it is not recommended that

analysts use the NNYFS public or

nonpublic data to produce household or

family-level estimates. Only limited

information on family and household

members’ relationships was collected in

NNYFS. The publicly released data do

not contain information on whether one

or more survey participants are related

or live in the same household. This

information, and information on whether

the participants live in the same

household, is only available through

RDC.

Geographic and Seasonal

Considerations

NNYFS interviewed and examined

a nationally representative sample of

Table J. Distribution of sample by race and Hispanic origin, NNYFS 2012

Race and Hispanic origin Number of interviewed Number of examined

(RIDRETH1) children (RIDSTATR = 1 or 2) children (RIDSTATR = 2)

Mexican American, regardless of race........ 
 

Other Hispanic, regardless of race .......... 
 

Non-Hispanic white, single race ........... 
 

Non-Hispanic black, single race ........... 
 

Other race (including multiracial persons)...... 
 

Total ............................ 
 

n (unweighted percent)

249 (15.2) 242 (15.4)

246 (15.0) 238 (15.1)

639 (39.0) 619 (29.2)

371 (22.6) 345 (21.9)

135 (8.2) 132 (8.4)

1,640 (100.0) 1,576 (100.0)

NOTES: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey. RIDSTATR is the interview

and examination status of the sample person.
Series 2, No. 163 [ Page 11

children located in 15 PSUs across the

country. This means that the sampled

children in these 15 PSUs represent the

approximately 3,000 counties in the

United States. No geographic location,

including true PSUs, is released on the

publicly available data files, in order to

protect the identification of NNYFS

respondents.

Due to operational considerations,

the geographical scheduling of the

NNYFS MEC was restricted by weather

considerations. MEC operations avoided

certain geographic areas during the

winter. Thus, the statistical efficiency of

the sample is diminished for any

variable that may be related to seasonal

variation that differs by region of the

country. In particular, consumption of

certain foods may be subject to

seasonality by geographical interaction.

Most NNYFS variables are not subject

to seasonality constraints. The variable

RIDEXMON, in the public release

demographic data file, provides the

6-month time period in which the

examination was performed (November

1 through April 30 and May 1 through

October 21).

Effect of Nonresponse

Like most population-based sample

surveys, NNYFS experienced both

participant (unit) and component (item)

nonresponse. In a statistical sense,

nonresponse can be considered ignorable

or nonignorable. If the data are missing

at random and the characteristics of the

nonrespondents are similar to the

characteristics of the respondents, the

nonresponse can be considered

ignorable. However, nonrespondents

may have characteristics significantly

different from respondents. In this case,

the nonresponse mechanism may be

nonignorable with respect to the data

analysis. Ignoring nonresponse in this

case leads to biased estimates.

Unit or sample child

nonresponse

All eligible children selected to

participate in NNYFS who completed

the household interview questionnaire

were defined as ‘‘interviewed,’’ and all

interviewed children who completed one

or more examination components in the

MEC were defined as ‘‘MECexamined.’’ Not all children in the

NNYFS sample were interviewed, and

not all interviewed children were

examined. Unit or sample child

nonresponse, which is the failure to

obtain any information on an individual

selected to participate in the NNYFS

survey, can occur at both the interview

and the examination phases of the

survey. Survey weights specific to the

interview and examination help account

for any loss due to nonresponse at these

stages of the survey.

The interview response rate for

NNYFS was 79.4%, and the

examination response rate (conditional

on the interview) was 96.1%. This

resulted in an overall examination

response rate of 76.3% (see also

http://www.cdc.gov/nchs/nnyfs/

response_rate_population_totals.htm).

However, not all the households selected

for NNYFS responded to the screening

interview. Applying the screener

response rate (98.5%) to the overall

examination response rate results in a

survey response rate of 75.2%.

Nonresponse bias resulting from

missing data can be an important source

of survey error. Nonresponse bias can be

substantial when two conditions hold:

(a) the response rate is relatively low

and (b) the difference between the

characteristics of respondents and

nonrespondents is relatively large. An

analysis was conducted to assess the

relationship between response status and

characteristics of the NNYFS sampled

children.

The NNYFS nonresponse bias

analysis was conducted in two stages.

An initial analysis involved the

comparison of selected demographic and

socioeconomic characteristics (e.g., age,

sex, race and Hispanic origin, general

health status, and median income of

geographic sampling unit) of

respondents with those of

nonrespondents. Further analyses were

then conducted, focusing on

nonresponse bias in final outcome

statistics. All analyses used design-based

methods for estimating variances to the

extent possible.

The initial analyses showed some

indication of potential for bias in the

respondent sample, prior to conducting

nonresponse adjustments, to the extent

that the characteristics analyzed are

related to health. However, several of

the characteristics found to be

significantly related to response status

were either used, or highly correlated

with those used, in the weighting

adjustments, indicating that the bias may

have been reduced through the

weighting adjustments.

To determine if any of the potential

bias identified in the analyses described

above remained after the weighting

adjustments, estimates of the

characteristics of selected persons from

the full sample (including

nonrespondents) were compared with

estimates for respondents only, before

and after weighting adjustments for

interviewed and examined persons. Very

few estimates indicated large relative

differences across the stages of

weighting for interviewed persons, and

none indicated large relative differences

for examined persons.

The nonresponse bias analyses

performed to date demonstrate potential

nonresponse bias before weighting

adjustments, but the weighting

adjustments reduced this bias. However,

the potential for nonresponse bias may

still exist because not all characteristics

could be evaluated as part of this

nonresponse bias analysis. The methods

and results of these and other

nonresponse bias analyses will be

presented in detail in a forthcoming

report.

Adjustments made for survey

nonresponse account only for interview

or examination nonresponse, but they do

not account for component or item

nonresponse (e.g., a child declined to

participate in the cardiovascular fitness

test but completed all other examination

components).

Component or item

nonresponse

There were several components in

the NNYFS examination (including

physical activity tests, tests of muscle

strength and endurance, body

measurements, and personal interviews),

and each component contained a number

of items. Some examinees may not have
Page 12 [ Series 2, No. 163

participated in all components for which Table K. Unavailable values in data, NNYFS 2012

they were eligible or may not have fully

completed a particular component, thus

resulting in component or item

nonresponse. These missing values may

distort analysis results.

Analysts must evaluate the extent of

missing data in the variables to be used

in their analyses to determine whether

the data are usable without additional

reweighting for item nonresponse. As a

general rule, if 10% or less of data for

the main outcome variable for a specific

component is missing for eligible

examinees, it is usually acceptable to

continue analysis without further

evaluation or adjustment. However, if

more than 10% of the data for a

variable is missing, the analyst may

need to further examine respondents and

nonrespondents with respect to the main

outcome variable and decide whether

imputation of missing values or use of

adjusted weights is necessary. It is

important to note that even if the overall

component is missing less than 10%, a

subgroup within the component may

exceed 10% and may need further

examination for statistical bias.

The component nonresponse varies

substantially by demographic

characteristics of the participants, the

type of component, and survey cycle.

Analysts are strongly encouraged to

examine component nonresponse to

determine whether the survey sample

weights need to be adjusted. See

previous NHANES publications where

component or item nonresponse

adjustment and reweighting have been

addressed (12,14).

Other key concepts about

missing data

NNYFS assigns missing values as

follows:

+ A period (.) for numeric variables

+ A blank for character variables

However, other types of data also

may be important to consider as

unavailable for analysis and part of

the unit nonresponse for that variable.

When a sample child refuses to answer

a question, a ‘‘refused’’ response is

Code Description Action

Period (.)

(Blank) ........................

7 or 77 or 777 ....................

9 or 99 or 999 ....................

Missing numeric value

Missing character value

‘‘Refused’’ response

‘‘Don’t know’’ response

None

None

Code as missing (period or blank)

Code as missing (period or blank)

NOTE: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey.

assigned a value of either ‘‘7,’’ ‘‘77,’’ or

‘‘777,’’ depending on the number of

digits in the variable value range. A

‘‘don’t know’’ response is assigned a

value of either ‘‘9,’’ ‘‘99,’’ or ‘‘999,’’

which is also dependent on the number

of digits in the variable value range; see

Table K.

Survey Sample Weights

The objective of NNYFS was to

produce data representative of

noninstitutionalized children in the

United States. The weighting of sample

data permits analysts to produce

estimates of statistics they would have

obtained if the entire sampling frame

had been surveyed. A sample weight is

assigned to each child. Sample weights

can be considered as measures of the

number of children represented by the

particular sampled child.

Weighting takes into account

several features of the survey: the

differential probabilities of selection for

each child, nonresponse to survey

instruments, and differences between the

final sample and the total population.

The sample weighting was

implemented in three steps. The first

step involved the computation of

weights to compensate for unequal

probabilities of selection for various age

groups. The second step adjusted for

participant nonresponse. Weights were

adjusted for nonresponse to the

household interview when creating the

interview weights and further adjusted

for nonresponse to the MEC

examination when creating the

examination weights. In the third step,

the sample weights were poststratified to

match U.S. Census Bureau estimates of

all noninstitutionalized children in the

United States. These steps were

performed for respondents at each stage

of the survey: the screener, personal

interview, and the examination.

The weighting methodology is

similar to that used for NHANES. A

more detailed discussion of the weights

produced for that survey can be found

in ‘‘The National Health and Nutrition

Examination Survey: Estimation

Procedures, 2007–2010’’ (14).

Determining the appropriate

sample weight for analysis

Three different sample weights are

available with the NNYFS data release.

Use of the correct sample weight for

NNYFS analyses is extremely important

and depends on the variables being

used. A good rule of thumb is to use

‘‘the least common denominator’’

approach. With this method, the analyst

checks the variables of interest. The

variable that was collected on the

smallest number of children is the least

common denominator, and the sample

weight that applies to that variable is the

appropriate one to use for that particular

analysis.

Any eligible child who did not

respond to the interview was assigned

an interview weight of zero (see ‘‘Unit

or sample child nonresponse’’ for overall

response rates). These children were

considered ineligible for the examination

and also were assigned an examination

weight of zero. Their records were not

included in the publicly released data.

Children who did complete the

interview and were eligible for the

examination but did not complete it

were assigned examination weights of

zero, and their records were included in

the public release. These cases, which

have a zero examination weight, should

be treated as missing when the
Series 2, No. 163 [ Page 13

Table L. Appropriate use of weights, NNYFS 2012

Weight	 Application

Interview weight........................ 	 Use when analyzing data from the home interview only. Do not use if the analysis also includes variables that

were collected on examined children in the MEC.

Examination weight. . . . . . . . . . . . . . . . . . . . . . 	 Use when analyzing data from the MEC examination. Do not use if the analysis includes variables collected as

part of the dietary interview.

Dietary weight ......................... 	 Use when analyzing data from the day 1, 24-hour dietary recall.
 

NOTES: NNYFS is National Health and Nutrition Examination Survey National Youth Fitness Survey. MEC is mobile examination center.

examination data are analyzed.Table L

lists the appropriate use of each set of

weights calculated for NNYFS.

Variance Estimation

The complex, multistage,

probability cluster design of NNYFS

will affect variance estimates (sampling

error). Typically, individuals within a

cluster (e.g., county, school, city, or

census block) are more similar to one

another than those in other clusters, and

this homogeneity of individuals within a

given cluster is measured by the

intracluster correlation. When working

with a complex sample, the ideal

situation is to limit the correlation

between sample persons within clusters.

This is accomplished by sampling fewer

people within each cluster but sampling

more clusters. However, because of

operational limitations similar to those

for NHANES (e.g., cost of moving the

survey MECs or geographic distances

between PSUs), NNYFS visited only 15

PSUs.

Variances should be calculated for

all survey estimates using the

appropriate methods for complex sample

surveys to aid in determining statistical

reliability. However, it is also important

to assess the reliability of the estimated

variances themselves.

Variance estimation methods

For complex sample surveys, exact

mathematical formulas for variance

estimation are usually not available.

Variance approximation procedures are

required to provide reasonable,

approximately unbiased, and designconsistent estimates of variance.

Variance estimates computed using

standard statistical software packages

that assume simple random sampling are

generally too low (i.e., significance

levels are overstated) and biased

because they do not account for the

differential weighting and the correlation

among sample persons within a cluster.

Two variance approximation procedures,

which account for the complex sample

design, are replication methods and

Taylor series linearization. Currently,

NCHS recommends the use of Taylor

series linearization for variance

estimation in NNYFS.

For either linearization or

replication, strata and PSU variables

must be available on the survey data

file. Because of confidentiality issues,

true PSUs cannot be released. In order

to use the Taylor series approach for

variance estimation, masked variance

units (MVUs) were created and

provided in the demographic data file.

The NNYFS MVUs are equivalent to

pseudo-PSUs used to estimate variance

in past NHANES surveys. MVUs on the

data file are not the ‘‘true’’ design PSUs.

They are a collection of secondary

sampling units aggregated into groups

for the purpose of variance estimation.

They produce variance estimates that

closely approximate the variances that

would have been estimated using the

‘‘true’’ design.

Software such as SUDAAN, Stata,

SPSS, and SAS survey procedures can

be used to estimate sampling errors by

the Taylor method. Software packages

that assume a simple random sample

should not be used for computing

variances for NNYFS. The stratum

(SDMVSTRA) and PSU (SDMVPSU)

needed for Taylor series linearization are

included in the demographic data file

for the NNYFS data release. See the

NHANES tutorial (http://www.cdc.gov/

nchs/tutorials/NHANES/preparing/

intro.htm) for more detail on software

programming code for analysis. Refer to

the NHANES estimation procedures

report (14) for more information on

variance estimation in NHANES

surveys.

Other sources of variability

As with any survey, quality control

procedures were established in NNYFS

to ensure that sources of error are

limited and that the data are of high

quality. It is inherent to any

measurement process that some sources

of variation cannot be controlled, and

users should be aware of these. Some

variables may be subject to withinrespondent variation. For example,

outcomes from a 24-hour dietary recall

questionnaire will not be the same if

taken on a different day. A child’s

measured performance on various

physical activity measures also may

vary as a result of fatigue or other

factors. By reading the data collection

protocols, users should be better able to

interpret NNYFS data relative to the

data collection procedures used.

Statistical Reliability of

Estimates

Issues of precision and statistical

reliability should be addressed for each

specific analysis. This is especially true

for NNYFS, given the small number of

PSUs in the sample. The statistical

reliability of an estimate depends on the

sample size on which it is based, the

relative standard error and the design

effect of the estimate, the reliability of

the estimated standard error, and

whether the estimate of interest is a rare

event or extreme proportion. Each of

these factors is described further in

‘‘National Health and Nutrition

Examination Survey: Analytic

Guidelines, 1999–2010’’ (12).

If an analysis results in an estimate

that cannot be considered reliable, a

reliable estimate may be obtained by
Page 14 [ Series 2, No. 163

collapsing subdomains to increase the

analytic sample size.

Estimation of Population

Counts

To understand the public-health

impact of a condition, it is often helpful

to calculate population counts in

addition to the prevalence of a health

condition. By quantifying the number of

people with a particular condition or

risk factor, counts directly speak to the

burden or magnitude of the condition.

There are a few basic steps in

calculating a population count for

NNYFS:

1.	 Calculate the unadjusted (crude)

prevalence of the health condition or

risk factor.

2.	 Use the relevant population totals

from the 2011 American Community

Survey (ACS) to determine

population estimates in NNYFS.

Because NNYFS is a nationally

representative survey of

noninstitutionalized children in the

U.S. population, population

estimates are based on the ACS

totals for this aspect of the U.S.

population. ACS-based population

tables for NNYFS by race and

ethnicity, sex, and age are located at

http://www.cdc.gov/nchs/nnyfs/

response_rate_population_totals.htm.

3.	 Multiply the prevalence of the risk

factor or health condition of interest

by the corresponding ACS-based

population total to obtain an estimate

of the number of noninstitutionalized

U.S. children with the risk factor or

condition. To calculate age-, sex-, or

race and ethnicity-specific

population estimates, multiply the

prevalence of the health condition in

each subdomain by the ACS

population total for the respective

subdomain.

4.	 Report population totals to the

nearest thousand with a 95%

confidence interval, based on the

95% confidence intervals computed

from the prevalence estimate.

Although the noninstitutionalized

ACS population totals are used to

calculate the final sampling weights for

NNYFS, the sum of the final sampling

weights for all sample children with the

risk factor or health condition of interest

cannot be used to arrive at population

estimates. The total population estimate

for a given risk factor or health

condition from the interviewed sample

should equal the sum of the final

interview weights for individuals with

that health condition. However, if there

are a significant number of exclusions

or missing data for a health condition,

summing the weights will not produce

an accurate population estimate.

Therefore, summing the final sampling

weights to arrive at population estimates

is not recommended.

Note that the population totals

generated in NNYFS can only be

representative of the number of children

with the health condition in the

noninstitutionalized U.S. population.

Combining NNYFS and

NHANES Data

The NNYFS sample was designed

so that data from components common

to both surveys in 2012, such as the

body measures, physical activity

monitor, muscle strength (grip strength),

physical activity questionnaire, dietary

recall, and specific household interview

questions could be analyzed together.

Therefore, data from NNYFS may be

combined with data from NHANES.

Combined weights for NNYFS and

NHANES 2012 (1-year weights) are

available through RDC (http://www.cdc.

gov/rdc/).

Information on the proposal process

and approval are available on the

NHANES website.

Conclusions

NCHS’ DHANES conducted

NNYFS in 2012, collecting the first

national data on physical activity

measures and physical fitness levels of

children and adolescents in the

United States since the 1980s. This

1-year survey provided data

representative of the U.S. youth

population aged 3–15 years, and it is

currently the only survey to obtain

direct physical measures in children and

adolescents on a national level. NNYFS

demonstrated that a survey of tests of

physical activity and fitness can be

conducted successfully in a limited

mobile environment. Together with

NHANES questionnaire and

examination data, NNYFS data will

provide a comprehensive picture of the

physical health of children in the

United States.

References

1.	 HHS. 2008 Physical Activity

Guidelines for Americans. Available

from: http://www.health.gov/

PAGuidelines/pdf/paguide.pdf.

2.	 Ogden CL, Carroll MD, Curtin LR,

Lamb MM, Flegal KM. Prevalence of

high body mass index in US children

and adolescents, 2007–2008. JAMA

303(3):242–9. 2010.

3.	 Iannotti RJ, Wang J. Trends in physical

activity, sedentary behavior, diet, and

BMI among US adolescents,

2001–2009. Pediatrics (132)4:606–14.

2013.

4.	 CDC. Youth Risk Behavior

Surveillance System (YRBSS).

Available from: http://www.cdc.gov/

HealthyYouth/yrbs/index.htm.

5.	 Trust for America’s Health. The

Affordable Care Act and the Prevention

and Public Health Fund: 2010

Allocations. Available from: http://

healthyamericans.org/assets/files/

The%20Affordable%20Care%20

Act%20And%20The%20Prevention

%20And%20PH%20Fund%202010

%20Allocations.pdf.

6.	 Zipf G, Chiappa M, Porter KS, et al.

The National Health and Nutrition

Examination Survey: Plan and

operations, 1999–2010. National Center

for Health Statistics. Vital Health Stat

1(56). 2013.

7.	 U.S. Census Bureau. Current

Population Survey (CPS)—Definitions.

Available from: http://www.census.gov/

cps/about/cpsdef.html.

8.	 PRO–ED, Inc. TGMD–2: Test of Gross

Motor Development (Second edition).

2000.

9.	 Canadian Fitness and Lifestyle

Research Institute. Canada Fitness

Survey Longitudinal Study: Reference

Booklet. 1988.

10.	 Canadian Society for Exercise

Physiology. The Canadian physical
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[ Page 15

activity, fitness& lifestyle approach.

Third edition. Ottawa, Canada:

Canadian Society for Exercise

Physiology. 2004.

11.	 The Cooper Institute. FitnessGram&

ActivtyGram test administration

manual. Champaign, IL: Human

Kinetics. 2010.

12.	 Johnson CL, Paulose-Ram R, Ogden

CL, et al. National Health and Nutrition

Examination Survey: Analytic

guidelines, 1999–2010. National Center

for Health Statistics. Vital Health Stat

2(161). 2013.

13.	 National Center for Health Statistics.

National Health and Nutrition

Examination Survey: Tutorials.

Available from: http://www.cdc.gov/

nchs/tutorials/.

14.	 Mirel LB, Mohadjer LK, Dohrmann

SM, et al. The National Health and

Nutrition Examination Survey:

Estimation procedures, 2007–2010.

National Center for Health Statistics.

Vital Health Stat 2(159). 2013.
Page 16 [ Series 2, No. 163

Appendix. Glossary

Ethics Review Board—In 2003, the

NHANES Institutional Review Board

changed its name to the NCHS Research

Ethics Review Board.

Household—The person or group of

persons living in an occupied dwelling

unit.

National Center for Health Statistics

(NCHS)—NCHS is the nation’s

principal health statistics agency. It

designs, develops, and maintains a

number of systems that produce data

related to demographic and health

concerns. These include data on

registered births and deaths collected

through the National Vital Statistics

System, the National Health Interview

Survey, the National Health and

Nutrition Examination Survey

(NHANES), the National Health Care

Surveys, and the National Survey of

Family Growth, among others. NCHS is

one of the institutes of the Centers for

Disease Control and Prevention, which

is part of the U.S. Department of Health

and Human Services.

Noninstitutional group quarters—

Group quarters that do not provide

formally authorized, supervised care or

custody in institutional settings. These

include college or university housing,

group homes intended for adults,

residential treatment facilities for adults,

workers’ group living quarters, Job

Corps centers, and religious group

quarters. Noninstitutional group quarters

are included in the NNYFS sample.

Noninstitutionalized civilian

population—Includes all people living

in households, excluding

institutionalized group quarters and

active-duty military personnel. This is

the target population for the NHANES

National Youth Fitness Survey

(NNYFS).

Segment—A group of housing units

located near one another, all of which

were considered for selection into the

sample. For NHANES, segments consist

of a census block or groups of blocks.

For NHANES, the selection of segments

comprises the second stage of sampling.

Within each segment, a sample of

dwelling units was selected.
Vital and Health Statistics

Series Descriptions

ACTIVE SERIES

Series 1.	 Programs and Collection Procedures—This type of report

describes the data collection programs of the National Center

for Health Statistics. Series 1 includes descriptions of the

methods used to collect and process the data, definitions, and

other material necessary for understanding the data.

Series 2.	 Data Evaluation and Methods Research—This type of

report concerns statistical methods and includes analytical

techniques, objective evaluations of reliability of collected

data, and contributions to statistical theory. Also included are

experimental tests of new survey methods, comparisons of

U.S. methodologies with those of other countries, and as of

2009, studies of cognition and survey measurement, and final

reports of major committees concerning vital and health

statistics measurement and methods.

Series 3.	 Analytical and Epidemiological Studies—This type of

report presents analytical or interpretive studies based on vital

and health statistics. As of 2009, Series 3 also includes

studies based on surveys that are not part of continuing data

systems of the National Center for Health Statistics and

international vital and health statistics reports.

Series 10.	 Data From the National Health Interview Survey—This

type of report contains statistics on illness; unintentional

injuries; disability; use of hospital, medical, and other health

services; and a wide range of special current health topics

covering many aspects of health behaviors, health status, and

health care utilization. Series 10 is based on data collected in

this continuing national household interview survey.

Series 11.	 Data From the National Health Examination Survey, the

National Health and Nutrition Examination Surveys, and

the Hispanic Health and Nutrition Examination Survey—

In this type of report, data from direct examination, testing,

and measurement on representative samples of the civilian

noninstitutionalized population provide the basis for (1)

medically defined total prevalence of specific diseases or

conditions in the United States and the distributions of the

population with respect to physical, physiological, and

psychological characteristics, and (2) analyses of trends and

relationships among various measurements and between

survey periods.

Series 13.	 Data From the National Health Care Survey—This type of

report contains statistics on health resources and the public’s

use of health care resources including ambulatory, hospital,

and long-term care services based on data collected directly

from health care providers and provider records.

Series 20.	 Data on Mortality—This type of report contains statistics on

mortality that are not included in regular, annual, or monthly

reports. Special analyses by cause of death, age, other

demographic variables, and geographic and trend analyses

are included.

Series 21.	 Data on Natality, Marriage, and Divorce—This type of

report contains statistics on natality, marriage, and divorce

that are not included in regular, annual, or monthly reports.

Special analyses by health and demographic variables and

geographic and trend analyses are included.

Series 23.	 Data From the National Survey of Family Growth—These

reports contain statistics on factors that affect birth rates,

including contraception and infertility; factors affecting the

formation and dissolution of families, including cohabitation,

marriage, divorce, and remarriage; and behavior related to

the risk of HIV and other sexually transmitted diseases.

These statistics are based on national surveys of women and

men of childbearing age.

DISCONTINUED SERIES

Series 4.	 Documents and Committee Reports—These are final

reports of major committees concerned with vital and health

statistics and documents. The last Series 4 report was

published in 2002. As of 2009, this type of report is included

in Series 2 or another appropriate series, depending on the

report topic.

Series 5.	 International Vital and Health Statistics Reports—This

type of report compares U.S. vital and health statistics with

those of other countries or presents other international data of

relevance to the health statistics system of the United States.

The last Series 5 report was published in 2003. As of 2009,

this type of report is included in Series 3 or another series,

depending on the report topic.

Series 6.	 Cognition and Survey Measurement—This type of report

uses methods of cognitive science to design, evaluate, and

test survey instruments. The last Series 6 report was

published in 1999. As of 2009, this type of report is included

in Series 2.

Series 12.	 Data From the Institutionalized Population Surveys—

The last Series 12 report was published in 1974. Reports

from these surveys are included in Series 13.

Series 14.	 Data on Health Resources: Manpower and Facilities—

The last Series 14 report was published in 1989. Reports on

health resources are included in Series 13.

Series 15.	 Data From Special Surveys—This type of report contains

statistics on health and health-related topics collected in

special surveys that are not part of the continuing data

systems of the National Center for Health Statistics. The last

Series 15 report was published in 2002. As of 2009, reports

based on these surveys are included in Series 3.

Series 16.	 Compilations of Advance Data From Vital and Health

Statistics—The last Series 16 report was published in 1996.

All reports are available online, and so compilations of

Advance Data reports are no longer needed.

Series 22.	 Data From the National Mortality and Natality Surveys—

The last Series 22 report was published in 1973. Reports

from these sample surveys, based on vital records, are

published in Series 20 or 21.

Series 24.	 Compilations of Data on Natality, Mortality, Marriage, and

Divorce—The last Series 24 report was published in 1996.

All reports are available online, and so compilations of reports

are no longer needed.

For answers to questions about this report or for a list of reports published

in these series, contact:

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