Number 65 n March 20, 2013

Changes in Prevalence of

Parent-reported Autism Spectrum Disorder in

School-aged U.S. Children: 2007 to 2011–2012

by Stephen J. Blumberg, Ph.D., Matthew D. Bramlett, Ph.D., National Center for Health Statistics;
 

Michael D. Kogan, Ph.D., Maternal and Child Health Bureau;
 

Laura A. Schieve, Ph.D., National Center on Birth Defects and Developmental Disabilities;
 

Jessica R. Jones, M.P.H., and Michael C. Lu, M.D., M.P.H., Maternal and Child Health Bureau
 

Abstract

Objectives—This report presents data on the prevalence of diagnosed autism

spectrum disorder (ASD) as reported by parents of school-aged children (ages

6–17 years) in 2011–2012. Prevalence changes from 2007 to 2011–2012 were

evaluated using cohort analyses that examine the consistency in the 2007 and

2011–2012 estimates for children whose diagnoses could have been reported in

both surveys (i.e., those born in 1994–2005 and diagnosed in or before 2007).

Data sources—Data were drawn from the 2007 and 2011–2012 National

Survey of Children’s Health (NSCH), which are independent nationally

representative telephone surveys of households with children. The surveys were

conducted by the Centers for Disease Control and Prevention’s National Center

for Health Statistics with funding and direction from the Health Resources and

Services Administration’s Maternal and Child Health Bureau.

Results—The prevalence of parent-reported ASD among children aged 6–17

was 2.00% in 2011–2012, a significant increase from 2007 (1.16%). The

magnitude of the increase was greatest for boys and for adolescents aged 14–17.

Cohort analyses revealed consistent estimates of both the prevalence of parentreported ASD and autism severity ratings over time. Children who were first

diagnosed in or after 2008 accounted for much of the observed prevalence

increase among school-aged children (those aged 6–17). School-aged children

diagnosed in or after 2008 were more likely to have milder ASD and less likely

to have severe ASD than those diagnosed in or before 2007.

Conclusions—The results of the cohort analyses increase confidence that

differential survey measurement error over time was not a major contributor to

observed changes in the prevalence of parent-reported ASD. Rather, much of the

prevalence increase from 2007 to 2011–2012 for school-aged children was the

result of diagnoses of children with previously unrecognized ASD.

Keywords: autism prevalence • pervasive developmental disabilities • national

estimates • State and Local Area Integrated Telephone Survey

Introduction

Autism spectrum disorder (ASD) is

a set of complex neurodevelopment

disorders that include autistic disorder,

Asperger disorder, and pervasive

developmental disorder not otherwise

specified (1). Children who have ASD

display mild to severe impairments in

social interaction and communication

along with restricted, repetitive, and

stereotyped patterns of behaviors,

interests, and activities. Diagnosis of

ASD should be based on comprehensive

behavioral evaluations, making

diagnostic assessment complex and

time-consuming.

ASD symptoms typically can be

identified in children as young as 18

months (2), and the American Academy

of Pediatrics recommends developmental

screening of all children by age 24

months (3). Nevertheless, many children

with ASD—especially those with only

mild or limited speech delays—may not

be diagnosed until they are of school

age, when parents become concerned

about an inability to make friends and

teachers notice difficulties with peer

interactions (3). Formal diagnoses may

also occur at this age because a named

disability (such as ASD) is needed for

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Center for Health Statistics 
Page 2 National Health Statistics Reports n Number 65 n March 20, 2013

school-aged children to qualify for

special education services under the

Individuals with Disabilities Education

Act (4).

The reported prevalence of ASD has

increased in recent decades. For

example, data from the Centers for

Disease Control and Prevention’s (CDC)

National Health Interview Survey

(NHIS) revealed a nearly fourfold

increase in parent-reported ASD

between the 1997–1999 and 2006–2008

surveillance periods (5), and CDC’s

Autism and Developmental Disabilities

Monitoring (ADDM) Network revealed

a 78% increase in ASD prevalence

between 2002 and 2008 (6).

This report is the first to estimate

the change in prevalence of parentreported ASD from 2007 to 2011–2012

and is based on newly released data

from the 2011–2012 National Survey of

Children’s Health (NSCH). Estimates

are presented for the prevalence and

severity of parent-reported ASD

diagnoses for school-aged children

(those aged 6–17 years). Detailed

analysis of ASD prevalence in this

group of children allowed for

comparisons with 2007 estimates for the

same birth cohort and an evaluation of

whether observed changes in prevalence

could be due to changes in survey-based

measurement error.

Methods

The data were drawn from the 2007

and 2011–2012 NSCH. NSCH is a

nationally representative survey

conducted by CDC’s National Center for

Health Statistics (NCHS) as a module of

the State and Local Area Integrated

Telephone Survey, with direction and

principal funding from the Health

Resources and Services Administration’s

Maternal and Child Health Bureau (7,8).

In both 2007 and 2011–2012, NSCH

was fielded as a random-digit-dial

telephone survey of households with

children aged 0–17 years in the

United States; the 2011–2012 NSCH

included both landlines and cell phones,

whereas the 2007 NSCH included

landlines only.

Contacted households were screened

for the presence of children, and one

child was randomly selected from

identified households with children to be

the subject of the survey. The

respondent was a parent or guardian in

the household who was knowledgeable

about the child’s health. These

respondents are referred to as ‘‘parents’’

throughout this report. A total of 91,642

NSCH 2007 interviews were completed

from April 2007 through July 2008. A

total of 95,677 NSCH 2011–2012

interviews were completed from

February 2011 through June 2012.

The overall response rate for

2011–2012 (23.0%) was lower than the

rate for 2007 (46.7%) primarily due to

the inclusion of cell-phone interviews in

2011–2012. Nonresponse bias analyses

suggest that, although the potential for

bias cannot be ruled out, differences

between respondents and

nonrespondents should not have a major

impact on the conclusions in this report.

Please see Technical Notes for details.

This report is based on parent report

of ASD diagnoses, ASD severity, age

when first diagnosed with ASD, and

approximate year of that first diagnosis.

Survey question wording and the

derivation of calculated variables are

described in the Technical Notes.

Children classified as having ASD were

those with a parent report of ever being

told by a doctor or other health care

provider that the child had ASD, and a

parent report that the child currently has

ASD. This measure of parent-reported

ASD is analogous to measures of

‘‘parent-reported ASD’’ (9) and ‘‘current

ASD’’ (10) used in previously published

reports on NSCH-based estimates of

ASD prevalence and trends.

Information about parent-reported

ASD diagnosis was obtained for 63,967

children aged 6–17 in 2007 and for

65,556 in 2011–2012. The cohort

analyses include information about

parent-reported ASD diagnosis for

56,399 children aged 2–13 in 2007.

A sampling weight was provided by

NCHS with the data record for each

child. This weight is based on the

probability of selection of the child’s

telephone number, with adjustments for

known survey response biases and

further adjustments to ensure that

weighted estimates match demographic

control totals from the U.S. Census

Bureau’s American Community Survey.

Estimates based on these weights,

including all national estimates produced

for this report, are representative of the

noninstitutionalized population of U.S.

children.

For more information about NSCH,

including its sample design, data

collection procedures, and questionnaire

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

slaits/nsch.htm.

Results

Based on parent reports, the

prevalence of diagnosed ASD in

2011–2012 was estimated to be 2.00%

for children aged 6–17. This prevalence

estimate (1 in 50) is significantly higher

than the estimate (1.16%, or 1 in 86) for

children in that age group in 2007.

As shown in Figure 1, statistically

significant increases in the prevalence of

parent-reported ASD were observed for

all age groups (from 1.31% to 1.82%

for ages 6–9; 1.45% to 2.39% for ages

10–13; and 0.73% to 1.78% for ages

14–17) and for boys (from 1.80% to

3.23%). The increase for girls (from

0.49% to 0.70%) was not statistically

significant. The increase in prevalence

was greater for boys (1.43 percentage

points) than girls (0.21 percentage

points) and greater for children aged

14–17 (1.05 percentage points) than for

children aged 6–9 (0.51 percentage

points).

In 2011–2012, school-aged boys

were more than 4 times as likely as

school-aged girls to have ASD (3.23%

compared with 0.70%). Comparisons of

the prevalence of ASD between the

narrower age groups (ages 6–9, 10–13,

and 14–17) that compose the school-age

group (ages 6–17) did not reveal any

significant differences in 2011–2012. In

contrast, age-related differences were

observed in 2007. At that time, children

aged 14–17 were less likely to have

ASD than children aged 10–13 (0.73%

compared with 1.45%).

Cohort analyses of

prevalence

The observed prevalence increases

in parent-reported ASD could be due to
National Health Statistics Reports n Number 65 n March 20, 2013 Page 3

SOURCE: CDC/NCHS, National Survey of Children’s Health, 2007 and 2011–2012. Percent 

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 2007 2011–2012 

6–17 6–9 10–13 14–17 Boys 6–17 Girls 6–17

Age in years 

1.16 

1.31 

1.45 

0.73 

1.78 1.80 

0.49 

2.00 

1.82 

2.39 

3.23 

0.70 

Figure 1. Percentage of children aged 6–17 years with parent-reported autism spectrum disorder, by age group and sex: United States,

2007 and 2011–2012

survey-based measurement changes over

time. One way to evaluate the potential

NOTES: ASD is autism spectrum disorder. NSCH is National Survey of Children’s Health. 

SOURCE: CDC/NCHS, National Survey of Children’s Health, 2007 and 2011–2012. Percent 

0.0 

0.5 

1.0 

1.5 

2.0 

NSCH 2011–2012, 

6–17 years 

NSCH 2007, 

2–13 years 

Approximate year of 

ASD diagnosis, in or 

after 2008 

Approximate year of 

ASD diagnosis, in or 

before 2007 

1.16 

1.37 

0.61 

Figure 2. Percentage of children born in 1994–2005 who have parent-reported autism

spectrum disorder, by survey and approximate year when child was first diagnosed with

ASD: United States, 2007 and 2011–2012

impact of measurement change is to

compare independent estimates from the

2007 and 2011–2012 NSCH survey

years for the same population (or birth

cohort) of children for diagnoses that

could have been reported in both survey

years. If these prevalence estimates from

2007 and 2011–2012 are identical, this

would suggest that the observed

prevalence increases are not due to

survey-based measurement error.

Children aged 6–17 in 2011–2012

were born in 1994–2005

(approximately) and would have been

about ages 2–13 in 2007 when the

previous NSCH was conducted. The

estimated prevalence of parent-reported

ASD for that birth cohort in 2007 was

1.16% (Figure 2).

The appropriate comparison group

for those who were aged 2–13 in 2007

would be those children aged 6–17 in

2011–2012 whose ASD diagnosis could

have been reported in 2007. That is, the

appropriate comparison group in

2011–2012 consists of children who

were first identified as having ASD in

or before 2007. The estimated
Page 4 National Health Statistics Reports n Number 65 n March 20, 2013

prevalence of parent-reported ASD for

children aged 6–17 in 2011–2012 whose

ASD was diagnosed in or before 2007

was 1.37% (Figure 2). This estimate is

statistically indistinguishable (p > 0.10)

from the estimate for that same birth

cohort from the 2007 NSCH, suggesting

that survey-based measurement error

was unlikely to have been a major

contributor to the observed prevalence

increases.

The results shown in Figure 2 and

detailed in Table 1 also suggest that the

observed increase in ASD prevalence

between 2007 and 2011–2012 among

children included in the 1994–2005 birth

cohort is related to relatively recent

diagnoses. The 0.61% of children aged

6–17 diagnosed with ASD in or after

2008 largely accounts for the prevalence

increase from 2007 to 2011–2012 for

this group.

Table 1 provides estimates for

similar comparisons within the three

narrower birth cohorts subsumed within

the total 1994–2005 birth cohort group.

For example, for children aged 6–9 in

2011–2012 (the cohort born in 2002–

2005), a comparison to the 2007

estimate for children from the same

birth cohort (that is, children aged 2–5

in 2007) illustrates that, for diagnoses

received in or before 2007, the 2011–

2012 ASD estimate (0.89%) was not

significantly different from the 2007

estimate (0.70%). Similar nonsignificant

differences were observed for children

aged 10–13 in 2011–2012 (1.68%

compared with 1.31%) and for children

aged 14–17 in 2011–2012 (1.53%

compared with 1.45%).

Table 1 also shows that, as

expected, relatively recent diagnoses

(that is, those occurring in or after 2008)

were more common for the cohorts of

children aged 6–9 (0.89%) and 10–13

(0.71%) in 2011–2012 than for the

cohort of children aged 14–17 in

2011–2012 (0.24%). These relatively

recent diagnoses largely account for the

prevalence increases from 2007 to

2011–2012 for these age groups.

Children diagnosed in or after 2008

represent 50.0%, 29.8%, and 13.5% of

children with parent-reported ASD who

were aged 6–9, 10–13, and 14–17 in

2011–2012, respectively.

Cohort analyses of severity

Cohort analyses can also be used to

examine the consistency of NSCH

estimates of the distribution of

children’s ASD severity, as judged by

their parents. The first two columns of

Figure 3 present a comparison of ASD

severity ratings between 2007 and

2011–2012 for the cohort of children

born in 1994–2005, where the 2011–

2012 estimates are restricted to those

children whose ASD diagnosis was first

received in or before 2007. The

consistency was confirmed: No

differences in ASD severity were

observed between children aged 2–13 in

2007 and children aged 6–17 in

2011–2012 who first received an ASD

diagnosis in or before 2007.

The second and third columns of

Figure 3 show the distribution of

parent-reported ASD severity levels by

the approximate year when the ASD

was diagnosed. Relative to other

NOTES: ASD is autism spectrum disorder. NSCH is National Survey of Children’s Health. 

SOURCE: CDC/NCHS, National Survey of Children’s Health, 2007 and 2011–2012. Percent 

0 

20 

40 

60 

80 

100 

Severe 

Moderate 

Mild 

Approximate year of ASD 

diagnosis, in or after 2008 

Approximate year of ASD 

diagnosis, in or before 2007 

Approximate year of ASD 

diagnosis, in or before 2007 

NSCH 2007, 2–13 years NSCH 2011–2012, 6–17 years 

48.7 49.5 

58.3 

35.1 33.6 

34.8 

16.3 16.9 

6.9 

Figure 3. Percent distribution of parent-rated severity of autism spectrum disorder for children born in 1994–2005 who currently have

autism spectrum disorder, by survey and approximate year when child was first diagnosed with ASD: United States, 2007 and 2011–2012
National Health Statistics Reports n Number 65 n March 20, 2013 Page 5

school-aged children with ASD in

2011–2012, those children aged 6–17

who were diagnosed in or after 2008

were more likely to have mild ASD and

less likely to have severe ASD, and

stratum-adjusted Cochran-MantelHaenszel tests reveal that this trend

toward less severe ASD for more recent

diagnoses was statistically significant.

Children aged 6–17 who were diagnosed

in or after 2008 were less than one-half

as likely as children diagnosed in or

before 2007 to have severe ASD (6.9%

compared with 16.9%).

Table 2 reveals that this trend is

most notable for older children. Among

children aged 14–17, almost none (1%)

of the children with diagnoses in or

after 2008 were classified as having

severe ASD, yet 18% of children whose

ASD diagnosis was received in or

before 2007 were classified as having

severe ASD.

Summary and

Discussion

Between 2007 and 2011–2012, the

prevalence estimate for parent-reported

ASD diagnoses among U.S. children

aged 6–17 increased significantly, from

1.16% to 2.00%. Increases were

observed in all age groups, and among

boys aged 6–17.

The increases in ASD prevalence

reported here extend an ongoing trend

observed in the United States and other

developed countries over the past

several decades (5,6,11). Assessment of

the NSCH age-specific changes in

parent-reported ASD prevalence reveals

two key components of this trend.

First, the prevalence of parentreported ASD for adolescents aged

14–17 in 2011–2012 was greater than

the prevalence for adolescents aged

14–17 in 2007. The observed increase in

this cross-sectional comparison is due

largely to differences between the two

birth cohorts that are being compared,

rather than a recent rise in new

diagnoses in adolescence. The birth

cohort that was aged 14–17 in 2007

(that is, children born in 1990–1993)

had the lowest prevalence of parentreported ASD among all birth cohorts in

2007, but as young adults aged 18–21 in

2011–2012, they were too old to be part

of the 2011–2012 survey. The birth

cohort that was aged 14–17 in 2011–

2012 (children born in 1994–1997) had

the highest prevalence of parent-reported

ASD in 2007, when they were aged

10–13. As adolescents aged 14–17 in

2011–2012, this birth cohort continued

to have a similarly high prevalence of

parent-reported ASD, and this

prevalence was significantly higher than

the observed prevalence for the birth

cohort that aged out of the survey.

Second, significant increases in

parent-reported ASD prevalence were

observed in both cross-sectional and

cohort analyses for children aged 6–9

and 10–13 in 2011–2012. While it is not

possible to definitively determine the

underlying reason for this trend, several

previous studies that also examined

changes over time in ASD prevalence

estimates within selected birth cohorts

suggest that increasing recognition of

children with ASD has had an important

impact (10,12,13). Evidence from

epidemiologic, genetic, and neuroscience

research points to the prenatal period as

the key exposure window for ASD risk

factors (14,15). If the causes of ASD are

related to factors that exist prior to or

occur just after birth, it is unlikely that

changes in ASD prevalence within the

cohorts of children aged 6–13 in

2011–2012 reflect ‘‘true’’ increases in

susceptibility to the condition at these

ages. Rather, changes in prevalence of

parent-reported ASD within these birth

cohorts (children born in 1998–2005)

likely reflect either changes in the

recognition of ASD by health

professionals or survey-based

measurement changes over time.

Closer examination of the parentreported data confirm that ASD

prevalence increases for the majority of

children were driven by new (post-2007)

diagnoses rather than changes in survey

reliability, such as changes in how

parents reported earlier (pre-2008)

diagnoses. The change (or lack thereof)

in how parents reported earlier

diagnoses was evaluated by comparing

2007 and 2011–2012 estimates for a

subgroup of the population limited to

children represented in both surveys

(that is, children from the same birth

cohorts) and limited to those ASD

diagnoses that ostensibly could have

been reported in both surveys (that is,

diagnoses first received in or before

2007). For this group of comparable

children, the prevalence estimate and

severity distribution from the 2011–2012

NSCH were not statistically different

from the prevalence estimate and

severity distribution from the 2007

NSCH. The 2007 and 2011–2012 NSCH

samples were independent crosssectional samples weighted to be

representative of the U.S. population of

noninstitutionalized children in the

respective survey years, and the slight

differences observed across surveys are

within the range of possible sampling

variation.

ASD prevalence increases due to

relatively recent diagnoses (that is, those

occurring in or after 2008) were most

common for the cohorts of children

aged 6–9 and 10–13 in 2011–2012, but

were also observed to a smaller degree

for children aged 14–17. Nearly

one-third of the school-aged children

reported to have ASD in 2011–2012

were reported to have been diagnosed in

or after 2008. While this is

understandable for many of the youngest

children (those aged 6–9), 30% of

children aged 10–13 and 14% of

children aged 14–17 were first

diagnosed in or after 2008 at age 7 or

over, well beyond the age when ASD

signs and symptoms should be clearly

notable. More than one-half of these

children were classified by their parents

as having ‘‘mild’’ ASD, and very few

were classified as having ‘‘severe’’ ASD.

Together, these findings suggest that the

increase in prevalence of parent-reported

ASD may have resulted from improved

ascertainment of ASD by doctors and

other health care professionals in recent

years, especially when the symptoms are

mild. Changes in the ascertainment of

ASD could occur because of changes in

ASD awareness among parents or health

care professionals, increased access to

diagnostic services, changes in how

screening tests or diagnostic criteria are

used, or increased special education

placements in the community.

The analyses in this report have

several strengths, including a large,
Page 6	 National Health Statistics Reports n Number 65 n March 20, 2013

nationally representative sample, with

sufficient sample size to explore ASD

within selected strata defined by age or

birth cohort. Major limitations of the

NSCH estimates are that they are

potentially subject to survey

nonresponse bias and that parents’

reports were not substantiated through

clinical evaluation or educational

records. Though the validity of parent

reports may be uncertain, other studies

reporting previous NSCH autism

estimates support their use. For

example, parent-reported autism

prevalence estimates from the 2003

NSCH were highly concordant with

NHIS estimates from the same time

period and were related as expected to

social, emotional, and developmental

difficulties experienced by children (16).

Moreover, NSCH estimates from the

2007 NSCH were comparable to the

ADDM Network estimates from the

same time period (6,9). The ADDM

Network tracks children aged 8 residing

in selected areas in the United States;

ASD case finding is through expert

clinical review of special education and

medical records from specialist

providers who serve children with

developmental disabilities. The 2011–

2012 NSCH estimates cannot be

compared in a similar manner because

the autism-focused questions in NHIS

differ substantially from those in the

recent NSCH, and ADDM data for

comparable years are not yet available.

Data from the ADDM Network

support the inference that there has been

an increase in ASD recognition by

community providers. While all children

defined as ASD cases in ADDM have

previously come to the attention of a

special education or health care provider

in their communities, not all of these

children had a previous diagnosis or

school classification specific to ASD.

Each surveillance year, a percentage of

children who had clear documentation

of autism traits in their records, but not

a specific ASD diagnosis or educational

placement, are nonetheless classified as

ASD cases by ADDM clinical

reviewers. This proportion of ‘‘ASD, but

no previous diagnosis’’ cases has

declined steadily as total ASD

prevalence rates have shown marked

increases (6).

In conclusion, the consistency in

independent estimates of parent-reported

ASD prevalence for children born in

1994–2005 and diagnosed in or before

2007 increases confidence that NSCH

data were not subject to differential

survey measurement error over time and

therefore can be used to monitor

changes in the prevalence of parentreported ASD over time. Increases in the

prevalence of parent-reported ASD

continued through 2011–2012, and much

of the recent increase—especially for

children aged 6–13—was the result of

diagnoses of children with previously

unrecognized ASD. A more detailed

report will further explore these findings

within sociodemographic subgroups and

with further consideration of the

relationship between health care system

factors and ASD case ascertainment.

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Page 8 National Health Statistics Reports n Number 65 n March 20, 2013

Table 1. Percentage of children with parent-reported autism spectrum disorder, by age cohort, approximate year of ASD diagnosis, and

survey: United States, 2007 and 2011–2012

NSCH 2007 NSCH 2011–2012

Year of diagnosis

Approximate birth year1

Child age

in years

Number of

children

with ASD

in sample

Diagnosis

in or

before

2007

Child age

in years

Number of

children

with ASD

in sample

In or

before

2007

In or

after

2008

Percent of

children

with ASD

diagnosed

in or

after 2008

1994–2005 ...............

2002–2005 ...............

1998–2001 ...............

1994–1997 ...............

2–13

2–5

6–9

10–13

672

162

259

251

1.16

0.70

1.31

1.45

6–17

6–9

10–13

14–17

1,393

440

513

440

1.37

0.89

1.68

1.53

0.61

0.89

0.71

0.24

30.8

50.0

29.8

13.5

1

Date of birth was not asked in the survey, so birth year was approximated from child’s age at interview.

NOTES: ASD is autism spectrum disorder. Age cohort is defined by child’s age at interview. Year of ASD diagnosis is approximated from date of interview, child’s age at time of interview, and

child’s age when first diagnosed with ASD. See Technical Notes for more detail. The National Survey of Children’s Health (NSCH) is a cross-sectional survey. Because of the random sampling

method, it is unlikely that any child was included in the survey in both 2007 and 2011–2012. Data are based on telephone interviews of parents or guardians of a sample of the noninstitutionalized

population of U.S. children. All population estimates are based on sampling weights assigned to the data for each child.

SOURCE: CDC/NCHS, National Survey of Children’s Health, 2007 and 2011–2012.
National Health Statistics Reports n Number 65 n March 20, 2013 Page 9

Table 2. Percent distribution of parent-reported severity of child’s autism spectrum disorder, by child’s age and approximate year of ASD

diagnosis: United States, 2011–2012

Approximate year of ASD diagnosis1

In or In or

Age of child in years, and severity, at time of interview before 2007 after 2008 All dates

6–17

Mild .................................................. 49.5 58.3 52.5

Moderate ............................................... 33.6 34.8 33.8

Severe ................................................ 16.9 *6.9 13.7

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100.0 100.0 100.0

6–9

Mild .................................................. 47.8 63.0 56.1

Moderate ............................................... 42.3 27.1 34.1

Severe ................................................ 9.9 *9.9 9.7

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100.0 100.0 100.0

10–13

Mild .................................................. 47.4 48.6 47.8

Moderate ............................................... 32.7 46.3 36.7

Severe ................................................ *19.9 *5.1 *15.5

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100.0 100.0 100.0

14–17

Mild .................................................. 52.8 69.3 55.1

Moderate ............................................... 29.6 29.5 29.6

Severe ................................................ 17.6 *1.2 15.3

Total . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100.0 100.0 100.0

* Estimate has a relative standard error greater than 30% and does not meet NCHS standards for reliability or precision. However, unreliable estimates can still differ significantly and reliably from

other estimates if the differences are sufficiently large.

1

Year of ASD diagnosis is approximated from date of interview, child’s age at time of interview, and child’s age when first diagnosed with ASD. See Technical Notes for more detail.

NOTES: ASD is autism spectrum disorder. Data are based on telephone interviews of parents or guardians of a sample of the noninstitutionalized population of U.S. children. All population

estimates are based on sampling weights assigned to the data for each child.

SOURCE: CDC/NCHS, National Survey of Children’s Health, 2011–2012.
Page 10 National Health Statistics Reports n Number 65 n March 20, 2013

Technical Notes

Response rates and analysis

of nonresponse

Response rates provide one measure

of the potential for nonresponse bias, the

possibility that the characteristics of the

sample interviewed differ from the

characteristics of the population of

interest in some meaningful way. The

2007 overall response rate was 46.7%

and the 2011–2012 overall response rate

was 23.0%. The reduction in the

response rate was due primarily to the

inclusion of cell-phone interviews in

2011–2012, which provided better

coverage of the population of children at

the expense of lower response rates. The

lower overall response rates for

cell-phone interviews were largely the

result of differences in the proportion of

telephone numbers that were not

answered and therefore provided no

indication of whether the number

belonged to a household. This

nonresolution rate was 19.1% for the

landline sample and 51.4% for the

cell-phone sample.

The resolution rate (i.e., the

complement of the nonresolution rate) is

one component of the overall response

rate, which is calculated as the product

of the resolution rate, the screening

completion rate (i.e., the proportion of

known households where the presence

or absence of children was successfully

determined), and the interview

completion rate [i.e., the proportion of

known households with children that

completed the National Survey of

Children’s Health (NSCH) interview].

Relative to the nonresolution rates, the

landline (54.1%) and cell-phone (41.2%)

interview completion rates were more

similar. For the landline sample, the

interview completion rate was

comparable to the 66.0% interview

completion rate from 2007.

In general, differences between

respondents and nonrespondents are

only weakly associated with response

rates for telephone surveys, and

meaningful nonresponse bias will exist

only when the likelihood of survey

response is related to the survey

variables of interest (17). Therefore,

regardless of the response rate, it is

recommended that survey researchers

evaluate the possible impact of such

differences on the quality of survey

estimates (18).

Nonresponse bias analyses were

conducted with the 2011–2012 NSCH

data (8) using several recommended

approaches (18). Analyses using

contextual information in the sampling

frame (such as sociodemographic

characteristics of the geographic areas

associated with landline telephone

numbers) suggested that response biases

in these characteristics would have little

effect on prevalence estimates of

parent-reported autism spectrum disorder

(ASD). However, another approach—

comparing ASD prevalence estimates for

responders who readily participated with

estimates for those who required more

effort to encourage participation—

revealed the possibility of a small effect.

Children whose parents participated

after five or more calls (1.58%) were

less likely to have parent-reported ASD

than those whose parents participated

with fewer calls (2.10%). If one

assumes that high-effort respondents are

similar to nonrespondents, this finding

would suggest that nonresponse may

have increased the 2011–2012 ASD

prevalence estimate by up to

0.16 percentage points. This assumption

cannot be validated with the current

data, but if true, this potential bias is

still only one-fifth of the magnitude of

the observed increase in parent-reported

ASD over time (0.84 percentage points)

and is smaller than potential sampling

error (the half-width of the 95%

confidence interval was 0.20 percentage

points).

Similar nonresponse bias analyses

are not available for prevalence

estimates from the 2007 NSCH, but it is

not unreasonable to expect that, as in

2011–2012, parents of children with

ASD might have been somewhat more

likely to readily respond to the survey.

Analyses for other health and health

care variables from the 2007 survey

indicate that estimated biases in these

survey estimates were small (within the

range of potential sampling error) and

inconsistent in direction depending on

the approach used to estimate bias (19).

Together, these results from the 2007

and 2011–2012 surveys suggest that any

differences between survey respondents

and survey nonrespondents should not

have had a major impact on the

conclusions in this report; however, the

potential for such impact cannot be

completely ruled out.

Impact of change in sample

design

The 2011–2012 NSCH sample

included both landlines and cell phones,

whereas the 2007 NSCH included

landlines only. This change in sample

design was necessitated by the increased

proportion of children living in cellphone-only households in 2011–2012

(20); however, this change prompts the

question of whether the observed

increase in prevalence of parent-reported

ASD may have been related to the

inclusion of cell-phone sample or to the

necessary changes to weighting

procedures to account for dual-frame

sampling. If ASD prevalence was higher

among children in cell-phone-only

households than among children in

households with landlines, then their

omission from the 2007 sample and

inclusion in the 2011–2012 sample

would have inflated the observed ASD

prevalence difference. However, the

ASD prevalence for children aged 6–17

in cell-phone-only households (1.91%)

in 2011–2012 was nonsignificantly

lower than for children in landline

households (2.10%). Furthermore,

unweighted analysis of the 2007 and

2011–2012 data for children aged 6–17

showed a 78% increase in ASD

prevalence, similar to the observed

increase of 72% based on analyses with

the sampling weights. This suggests that

changes in the weighting procedures due

to dual-frame sampling were unlikely to

have contributed to the observed

increase.

Definition of terms

Autism spectrum disorder (ASD)—

Children with parent-reported ASD were

identified based on parents’ or

guardians’ responses to two questions.
National Health Statistics Reports

n Number 65n March 20, 2013 Page 11

First, parents were asked if they had

ever been told bya doctor or other

health care provider that their child had

‘‘autism, Asperger’s disorder, pervasive

developmental disorder, or other autism

spectrum disorder.’’ If parents responded

affirmatively, they were asked: ‘‘Does

[child] currently have autism or autism

spectrum disorder?’’ Children classified

as having ASD were those witha parent

report of (a) ever being told bya doctor

or other health care provider that their

child had ASD and (b) the child

currently having ASD. These two

questions were included in both the

2007 and 2011–2012 NSCH.

Age at diagnosis—In the 2011–2012

NSCH, parents or guardians who

reported ever being told bya doctor or

other health care provider that their

child had ASD were asked: ‘‘How old

was [child] when you were first told by a doctor or other health care provider

that [he/she] had autism or autism

spectrum disorder?’’ This question was

preceded bya reminder about the

definition of ASD: ‘‘Earlier you told me

that [child] has been diagnosed with

autism or an autism spectrum disorder,

such as Asperger’s disorder or pervasive

developmental disorder.’’ The question

about age at diagnosis was asked after

the parent reported that the child had

ever been diagnosed and before the

parent was asked if the child currently

had ASD. This question did not appear

on the 2007 NSCH.

Approximate year of diagnosis—For

the 2011–2012 NSCH, the year when

the child was first identified as having

ASD was approximated using data on

the child’s age at diagnosis, the child’s

age at the time of the interview, and the

date of the interview. The difference

between the age at interview (in years)

and the age at diagnosis (in years) was

calculated. If the survey was completed

in 2011 and the difference was3 years

or less, or if the survey was completed

in 2012 and the difference was4 years

or less, then the year when the child

was first diagnosed was calculated as in

or after 2008. Otherwise, the year the

child was first diagnosed was calculated

as in or before 2007.

The 2007 NSCH did not includea

question about age at diagnosis. All

children reported to have ASD in the

2007 NSCH were assumed to havea

year of diagnosis that was in or before

2007; that is, the year of diagnosis was

assumed to be in or before the year of

the survey.

Severity of ASD—Parents or

guardians who reported that the child

currently has ASD were asked to

providea qualitative ranking of severity:

‘‘Would you describe [his/her] autism or

autism spectrum disorder as mild,

moderate, or severe?’’
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National Health Statistics Reports n Number 65 n March 20, 2013

Acknowledgment

NCHS thanks Benjamin J. Skalland and Kathleen B. Santos from NORC at the University of Chicago for providing the analysis of nonresponse

included in this report.

Suggested citation

Blumberg SJ, Bramlett MD, Kogan MD, et al.

Changes in prevalence of parent-reported

autism spectrum disorder in school-aged U.S.

children: 2007 to 2011–2012. National health

statistics reports; no 65. Hyattsville, MD:

National Center for Health Statistics. 2013.

Copyright information

All material appearing in this report is in the

public domain and may be reproduced or

copied without permission; citation as to

source, however, is appreciated.

National Center for Health Statistics

Edward J. Sondik, Ph.D., Director
 

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

for Science
 

Division of Health Interview Statistics

Jane F. Gentleman, Ph.D., Director

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DHHS Publication No. (PHS) 2013–1250 • CS238979