Reliability and Validity of the Yale Global Tic Severity Scale
Eric A. Storch, Tanya K. Murphy, Gary R. Geffken,
Muhammad Sajid, and Pam Allen
University of Florida
Jonathan W. Roberti
New College of Florida
Wayne K. Goodman
University of Florida
To investigate the reliability and validity of the Yale Global Tic Severity Scale (YGTSS), 28 youth aged
6 to 17 years with Tourette’s syndrome (TS) participated in the study. Data included clinician reports of
tics and obsessive–compulsive disorder (OCD) severity, parent reports of tics, internalizing and exter-
nalizing problems, and child reports of depression and anxiety. All children participated in a 2nd YGTSS
administration by the same rater 48 days later. Good internal consistency and stability were found for the
YGTSS scores. YGTSS scores demonstrated strong correlations with parent-rated tic severity (r ?
.58–.68). YGTSS scores were not significantly related to measures of clinician ratings of OCD severity
(r ? .01–.15), parent ratings of externalizing and internalizing behavior (r ? ?.07–.20), and child ratings
of depression (r ? .02–.26) and anxiety (r ? ?.06–.28). Findings suggest that the YGTSS is a reliable
and valid instrument for the assessment of pediatric TS.
Keywords: Tourette’s syndrome, Yale Global Tic Severity Scale, clinician-rated, validity, reliability
Gilles de la Tourette syndrome, or Tourette’s syndrome (TS), is
characterized by involuntary motor and vocal tics that last over 1
year and begin in childhood or early adolescence (American Psy-
chiatric Association, 2000; Leckman & Cohen, 1999; World
Health Organization, 2000). Motor tics are repetitive, automatic,
and rapid movements of a complex or simple nature that affect
multiple muscle groups. Vocal tics are varied and include sniffing,
throat clearing, grunting, and repeating words or phrases. Child-
hood TS runs a waxing and waning course and is often comorbid
with other psychiatric conditions, including attention-deficit/hy-
peractivity disorder (ADHD; Comings, 1994; Robertson, Baner-
jee, Eapen, & Fox-Hiley, 2002; Sukhodolsky et al., 2003),
obsessive–compulsive disorder (OCD; Hebebrand et al., 1997;
Pauls, Towbin, Leckman, Zahner, & Cohen, 1986; Robertson,
2003), disruptive behavior (Budman, Bruun, Park, Lesser, & Ol-
son, 2000; Riddle, Hardin, Ort, Leckman, & Cohen, 1988; Sukh-
odolsky et al., 2003), and depressive and anxiety disorders (Carter
et al., 2000; Coffey, Biederman, Smoller, et al., 2000; Coffey &
Park, 1997; Robertson et al., 2002).
Although clinicians and researchers have uniformly recognized
the importance of assessing tic severity among youth, it has been
measured differently across instruments. The Tourette Syndrome
Severity Scale (TSSS; Shapiro & Shapiro, 1984) contains five
ordinal scales with differing ranges and item weights that focus on
TS-related social impairment. The Tourette Syndrome Global
Scale (TSGS; Harcherik, Leckman, Detlor, & Cohen, 1984) as-
sesses the frequency and disruption of simple and complex tics, as
well as common comorbid problems (e.g., behavioral problems,
functional impairment). Finally, short, structured videotape proto-
cols have been used to count tics (Shapiro & Shapiro, 1984;
Tanner, Goetz, & Klawans, 1982). Although these instruments
represent important advancements in pediatric TS assessment,
numerous concerns about their use have been raised, including
issues with scoring structures, practicality and efficiency, and
limited psychometric support (Leckman et al., 1989).
To address these limitations, Leckman and colleagues (1989)
developed the Yale Global Tic Severity Scale (YGTSS). The
YGTSS is a semistructured clinician-rated instrument that assesses
the nature of motor and phonic tics over the previous week. The
clinician initially notes the presence of motor and phonic tics based
on child and parent(s) reports and behavioral observations. Fol-
lowing this, the clinician rates the severity of motor and phonic tics
on five separate dimensions each: number, frequency, intensity,
complexity, and interference. The YGTSS also includes a separate
impairment rating focusing on distress and impairment experi-
enced in interpersonal, academic, and occupational realms. Five
index scores are obtained: Total Motor Tic Score, Total Phonic Tic
Score, Total Tic Score, Overall Impairment Rating, and Global
Severity Score. The Total Motor Tic Score is derived by adding the
five items pertaining to motor tics (range ? 0–25); the Total
Phonic Tic Score is derived by adding the five items pertaining to
phonic tics (range ? 0–25); the Total Tic Score is derived by
Eric A. Storch, Department of Pediatrics, University of Florida, and
Department of Psychiatry, University of Florida; Tanya K. Murphy, Gary
R. Geffken, Muhammad Sajid, Pam Allen, and Wayne K. Goodman,
Department of Psychiatry, University of Florida; Jonathan W. Roberti,
Department of Psychology, New College of Florida.
This research was funded by grants from the National Institute of Mental
Health to Tanya K. Murphy (Grant K23 MH01739) and Wayne K. Good-
man (Grant R01 MH45802). We thank Audrey Baumeister, Paula Edge,
Natalie Johns, Erin Killiany, Jane Mutch, and Ohel Soto for their contri-
butions to this study.
Correspondence concerning this article (and requests for an extended
report of this study) should be addressed to Eric A. Storch, Department of
Psychiatry, University of Florida, Box 100234, Gainesville, FL 32610.
2005, Vol. 17, No. 4, 486–491
Copyright 2005 by the American Psychological Association
adding the Total Motor Tic Score and the Total Phonic Tic Score;
and the Overall Impairment Rating is rated on a 50-point scale
anchored by 0 (no impairment) and 50 (severe impairment). A
Global Severity Score is derived by summing the Total Motor Tic
Score, Total Phonic Tic Score, and Overall Impairment Rating
(range ? 0–100).
Despite being widely used, only one psychometric investigation
has been published on the YGTSS (Leckman et al., 1989). This
sample included 105 children and adults (age range ? 5–51 years)
with a tic disorder. Good interrater agreement among three raters
was found for the YGTSS scores in a sample of 20 children and
adolescents with TS (with intraclass correlation coefficients
[ICCs] for index scores ranging from .62 to .85). Walkup, Rosen-
berg, Brown, and Singer (1992) corroborated this finding in a
second sample of 20 children and adolescents (the average Spear-
man rank coefficient among three raters for YGTSS Global Se-
verity Score was .93). Convergent validity was supported as
YGTSS scores were strongly related to corresponding TSGS
scores (Pearson product–moment correlations ranged from .86 to
.90). In addition, YGTSS scores were moderately to strongly
related to clinician ratings of TS impairment (Pearson correlations
ranged from .65 to .82) and moderately to strongly related to the
TSSS (Pearson correlations ranged from .54 to .76). Discriminant
validity was demonstrated by weak to moderate relations with
clinician ratings of impairment due to ADHD (Pearson correla-
tions ranged from ?.03 to .18) and OCD (Pearson correlations
ranged from .30 to .39). Finally, studies have demonstrated that the
YGTSS is sensitive to treatment effects (Gaffney et al., 2002;
Gilbert, Batterson, Sethuraman, & Sallee, 2004; Gilbert et al.,
2003; Gilbert, Sethuraman, Sine, Peters, & Sallee, 2000; Mueller-
Vahl et al., 2003; Scahill, Leckman, Schultz, Katsovich, & Peter-
Although encouraging, the following important psychometric
questions have not been examined: What is the stability and
internal consistency of YGTSS scores, and how does the YGTSS
relate to parent-rated TS symptoms and measures of anxiety and
depression? That the stability has not been reported is notable in
light of recent data suggesting that scores on the second adminis-
tration of clinician-rated instruments are often lower than on the
first administration (Arrindell, 2001; Jensen, Roper, Fisher, &
Piacentini, 1995). The implications of this finding are significant
and may suggest that observed treatment effects are actually due to
the retest effect rather than the impact of the treatment. Given this,
it is important to document potential temporal fluctuations in
YGTSS scores. In light of the high comorbidity with internalizing
and externalizing disorders, along with the frequency with which
child measures do not differentiate between disorders (Schniering,
Hudson, & Rapee, 2000), it is also important to examine the extent
to which the YGTSS taps unique constructs.
This research examined the psychometric properties of the
YGTSS. The following questions were addressed: (a) What are the
internal consistency and interscale correlations of the YGTSS? (b)
What is the stability of the YGTSS over a period of approximately
7 weeks? And (c) does the YGTSS correlate strongly with a
measure of TS symptomatology, and weakly with measures of
A total of 28 (14 female and 14 male) children diagnosed with TS
according to DSM–IV–TR (American Psychiatric Association, 2000) par-
ticipated in this research. Participants were referred from area physicians
and the University of Florida child psychiatry clinic. Diagnoses were
derived from the Schedule for Affective Disorders and Schizophrenia for
School-Age Children—Lifetime and Present versions (K-SADS–PL; Kauf-
man et al., 1997) and confirmed by Tanya K. Murphy, who was a board
certified child psychiatrist with 10 years of experience. The sample was
composed primarily of White participants (94.5%), followed by Latino
American (4.5%). At the initial assessment, the children ranged in age from
6 to 17 years with a mean age of 10.47 years (SD ? 2.51 years).
Participants generally came from middle to higher socioeconomic status
families (mean income ? $116,607), although there was fair variation
(SD ? $81,996, range ? $20,000–$340,000). All of these children were
being treated pharmacologically at study onset with the most common
medications including selective serotonin reuptake inhibitors, clonidine,
guanfacine neuroleptics, and stimulants. Secondary diagnoses, when
present, included ADHD (n ? 6), OCD (n ? 14), major depression (n ?
2), generalized anxiety disorder (GAD; n ? 1), oppositional defiant dis-
order (ODD; n ? 3), and social phobia (n ? 1). Tertiary diagnoses, when
present, included ADHD (n ? 4), GAD (n ? 3), ODD (n ? 1), and conduct
disorder (n ? 1).
sure were previously discussed.
Children’s Yale–Brown Obsessive Compulsive Scale (CY-BOCS; Scahill
et al., 1997).
The CY-BOCS is a 10-item semistructured clinician-
administered measure of obsession and compulsion severity. Items are
rated over the previous week on a 5-point Likert-type scale ranging from
0 to 4, with higher scores corresponding to greater symptom severity. Items
about obsessions and compulsions are summed to derive the Obsession and
Compulsion Severity Scales, respectively. The Obsession and Compulsion
Severity Scales are summed to derive the Total Severity Score. Only the
Total Severity Score was used in this study. The CY-BOCS has exhibited
good internal consistency (? ? .90), test–retest reliability over 6 weeks
(Total Score ICC ? .79), and convergent and discriminant validity (Scahill
et al., 1997; Storch et al., 2004b). Cronbach’s alpha for the Total Severity
Score in this sample was .94.
Tourette’s Disorder Scale—Parent Rated (TODS–PR; Shytle et al.,
The TODS–PR is a 15-item parent-rated scale designed to mea-
sure a broad range of symptoms common to Tourette’s disorder including
tics, obsessions, compulsions, inattention, hyperactivity, aggression, and
mood disturbances. The TODS–PR has four factorially derived subscales:
Aggression, ADHD, Obsessive–Compulsive Disorder/Anxiety, and Tics.
Internal consistency of the TODS–PR Total Score and factors has been
found to be adequate: TODS–PR Total Score (15 items; .91), Tics factor (2
items; .64), Aggression factor (5 items; .91), ADHD factor (4 items; .91),
and OCD/Anxiety factor (4 items; .81). In addition, the TODS–PR has
demonstrated good convergent and discriminant validity (Shytle et al.,
2003; Storch et al., 2004a). Cronbach’s alpha in this sample for the Tics,
Aggression, ADHD, and OCD factors were .69, .90, .92, and .74,
Children’s Depression Inventory (CDI; Kovacs, 1981).
widely used 27-item self-report measure that yields a severity rating of
depressive symptomatology across affective, cognitive, somatic, and be-
havioral domains. On each item, the child is instructed to choose one of
three statements that best describes his or her feelings over the past 2
weeks. Good psychometric properties have been reported including ade-
quate internal consistency and test–retest reliability (Smucker, Craighead,
The characteristics and psychometric properties of this mea-
The CDI is a
Craighead, & Green, 1986) and construct validity (Kovacs, 1992; Saylor,
Finch, Spirito, & Bennett, 1984). Cronbach’s alpha in this sample was .86.
The Multidimensional Anxiety Scale for Children (MASC; March,
Parker, Sullivan, Stallings, & Conners, 1997).
self-report questionnaire that assesses symptoms of general, social, and
separation anxiety in children and adolescents. Items are rated on a 4-point
Likert-type scale (0 ? never true about me, 1 ? rarely true about me, 2 ?
sometimes true about me, 3 ? often true about me), and a total score is
computed by summing all items. The MASC has good internal consistency
(? ? .90) and test–retest reliability over intervals of 3 weeks and 3 months
(r ? .88 and .87; March et al., 1997; March, Sullivan, & Parker, 1999). The
convergent and discriminant validity of the measure was supported, as the
MASC correlated moderately with the Revised Children’s Manifest Anx-
iety Scale (Reynolds & Richmond, 2000) (r ? .63) and was not signifi-
cantly correlated with the CDI (Kovacs, 1992) (r ? .19) or the Abbreviated
Symptom Questionnaire (Goyette, Conners, & Ulrich, 1978) (r ? .07).
Cronbach’s alpha in this sample was .90.
The MASC is a 39-item
Following permission from the University of Florida institutional review
board, parental consent and child assent were obtained. After this, children
and their parent(s) were taken to a private, secure office and administered
the K-SADS-PL to establish DSM–IV diagnoses. A board-certified child
psychiatrist (Tanya K. Murphy) confirmed diagnoses based on all available
clinical information. Following administration of the K-SADS-PL, a dif-
ferent clinician administered the YGTSS and CY-BOCS to the parent(s)
and child jointly. Procedures for the YGTSS interviewer training were
described in Leckman et al. (1989); procedures for CY-BOCS training
were described in Scahill et al. (1997). Clinicians were three licensed
psychiatrists and one psychiatric nurse with extensive clinical experience
working with pediatric TS. Training consisted of an instructional meeting
about the YGTSS content and structure with Tanya K. Murphy.
Following administration of the structured interviews, children com-
pleted the MASC and CDI, and parents completed the TODS–PR. All
measures were completed in a private room. To examine the stability of the
YGTSS over 47.8 days (SD ? 21.4; range ? 27 to 104 days), a different
clinician readministered the YGTSS using identical procedures. This time
distribution was primarily due to scheduling difficulties.
Cronbach’s alpha (Cronbach, 1951) was computed to assess the internal
consistency of the YGTSS scores at both the first and second administra-
tion. To assess test–retest reliability across the first and second YGTSS
administrations, one-way random effects ICCs (Shrout & Fleiss, 1979)
were used to allow for intersubject variability. ICC is a measure of
agreement for dimensional measurements. Scores range between 0 and 1,
with scores greater than .75 indicating excellent reliability (Nunnally &
Bernstein, 1994). Pearson product–moment correlations were computed to
examine the relationships among YGTSS scores and measures of TS
symptomatology, externalizing behaviors, OCD, anxiety, and depressive
Cronbach’s alpha reliability coefficients were high for the Total
Motor Tic Score (? ? .92 and .92), Total Phonic Tic Score (? ?
.93 and .93), and Total Tic Score (? ? .93 and .94) at first and
At the first assessment, the Total Motor Tic Score was moder-
ately correlated with the Total Phonic Tic Score and YGTSS
Overall Impairment Rating. The first administration Total Phonic
Tic Score and YGTSS Overall Impairment Rating were strongly
correlated (see Table 1). Similar results were found among second
Descriptive Statistics and Intercorrelations Between the YGTSS Scores and Various Measures of Psychological Functioning
1. YGTSS Motor Score
2. YGTSS Phonic Score
3. YGTSS Total Tic Score
4. YGTSS Impairment score
5. YGTSS Total Score
6. TODS-PR Tic
7. TODS-PR Aggression
8. TODS-PR ADHD
9. TODS-PR OCD
10. CY-BOCS Total
11. CDI Total
12. MASC Total
Phonic Tic Score; YGTSS Total Score ? Yale Global Tic Severity Scale Global Severity Score; YGTSS Impairment Score ? Yale Global Tic Severity
Scale Overall Impairment Score; TODS-PR Tic ? Tourette’s Disorder Scale—Parent Rated Tics factor; TODS-PR Aggression ? Tourette’s Disorder
Scale—Parent Rated Aggression factor; TODS-PR ADHD ? Tourette’s Disorder Scale—Parent Rated ADHD factor; TODS-PR OCD ? Tourette’s
Disorder Scale—Parent Rated Obsessive—Compulsive Disorder/Anxiety factor; CY-BOCS Total ? Children’s Yale-Brown Obsessive Compulsive Scale
Total Score; CDI Total ? Children’s Depression Inventory Total Score; MASC Total ? Multidimensional Anxiety Scale for Children Total Score.
* p ? .01. ** p ? .001.
YGTSS Motor Score ? Yale Global Tic Severity Scale Total Motor Tic Score; YGTSS Phonic Score ? Yale Global Tic Severity Scale Total
The stability of the YGTSS scores over 47.8 days (SD ? 21.4)
were examined using one-way random effects ICCs. ICCs ranged
from fair to excellent for the Total Motor Tic Score (ICC ? .77;
95% confidence interval [CI] ? .49 to .89), Total Phonic Tic Score
(ICC ? .90; 95% CI ? .78 to .95), Total Tic Score (ICC ? .88;
95% CI ? .73 to .94), Overall Impairment Rating (ICC ? .88;
95% CI ? .74 to .95), and Global Severity Score (ICC ? .89; 95%
CI ? .75 to .95). ICCs were also computed separately for partic-
ipants (n ? 19) whose readministration interval was below 47.8
days. For this subsample, stability ranged from fair to excellent for
the Total Motor Tic Score (ICC ? .85; 95% CI ? .62 to .94), Total
Phonic Tic Score (ICC ? .93; 95% CI ? .83 to .97), Total Tic
Score (ICC ? .92; 95% CI ? .78 to .96), Overall Impairment
Rating (ICC ? .88; 95% CI ? .71 to .95), and Global Severity
Score (ICC ? .91; 95% CI ? .78 to .96). Stability was not
calculated for participants whose administration interval was
above 47.8 days because of the small number of such participants
(n ? 9).
Convergent and Discriminant Validity
Table 1 presents Pearson product–moment correlations for chil-
dren with TS between the YGTSS scores and measures of parent-
rated tics, internalizing and externalizing symptoms, clinician-
rated OCD, and child-rated depression and anxiety. Correlations
were strong between all of the YGTSS scores and the TODS–PR
Tic factor. Discriminant validity was demonstrated with weak,
nonsignificant correlations between the YGTSS scores and CY-
BOCS Total, TODS–PR Aggression, ADHD, and OCD scores,
CDI Total, and MASC Total.
This study examined the psychometric qualities of the YGTSS
in children and adolescents with TS. Unlike other instruments that
measure pediatric TS, the YGTSS contains separate scales to
assess motor and phonic tics, as well as tic-related impairment.
Within clinical trials, the YGTSS represents the “reference stan-
dard” in pediatric tic assessment. However, limited psychometric
data have been reported, particularly regarding the stability of
scores, internal consistency, and convergent and discriminant
Overall, the results of the present study support both the reli-
ability and the validity of the YGTSS. The internal consistency of
the Total Motor Tic Score, Total Phonic Tic Score, and Total Tic
Score were high at both the first and second administrations. The
moderate level of intercorrelation between the Total Motor and
Phonic Tic Scores suggests that each measures relatively distinct
but related dimensions of TS. Of interest, the Total Phonic Tic
Score was more strongly related to the impairment index than the
Total Motor Tic Score. There are several interpretations of this
finding. First, phonic tics are generally understood to be more
impairing than motor tics alone by virtue of simultaneous motor
and phonic tics. Second, motor tics may be more easily hidden
than phonic tics. Attention or disruption resulting from phonic tics
may invite peer ridicule and/or reprimands should someone not
understand the nature of TS.
A primary goal of this study was to report data on the stability
of YGTSS scores. Others have cited the retest effect to be a source
of unreliability in symptom measurement that may have significant
implications for measuring treatment effects (Arrindell, 2001;
Jensen et al., 1995; Storch, Masia-Warner, Dent, Roberti, &
Fisher, 2004). Our findings suggest that the retest effect was not
substantial in YGTSS scores for this sample, as scores were
relatively consistent across administrations even with children
being maintained on their prestudy pharmacological regimen. It is
worth noting that the use of different raters at each time point
provides for a more stringent test of stability and helps to control
for rater bias.
The convergent validity of the YGTSS scores was supported by
strong correlations with parent reports of tics. Although the high
relationships suggest that parent ratings of tics may be an easier
manner of collecting information, the YGTSS holds numerous
advantages over parent reports. For example, YGTSS scores are
based on all available information (e.g., parent and child reports,
behavioral observations) as opposed to parent ratings in isolation.
In addition, the YGTSS provides a qualitatively rich account of the
type and nature of symptoms. The pattern of weak relations with
measures of externalizing behaviors, OCD, anxiety, and depres-
sion provides strong discriminant validity support. The demon-
strated divergence with these measures is noteworthy given the
high comorbidity with externalizing symptoms (Budman et al.,
2000; Comings, 1994; Riddle et al., 1988; Robertson et al., 2002;
Sukhodolsky et al., 2003), internalizing symptoms (Carter et al.,
2000; Coffey, Biederman, Geller, et al., 2000; Coffey, Biederman,
Smoller, et al., 2000; Coffey & Park, 1997; Robertson et al., 2002),
and OCD (Hebebrand et al., 1997; Pauls et al., 1986; Robertson,
Several limitations of this study should be noted. First, our
sample was small and consisted of primarily Caucasian children.
Further, complete demographic information was not available for
this sample. It is not clear whether our findings can be generalized
to other populations with differing demographic characteristics.
The small sample also prevents us from analyzing the YGTSS
dimensional structure. Indeed, the high relationship of the Total
Phonic Tic Score to the Total Tic score (.92), Overall Impairment
Rating (.91), and Global Severity Rating (.95) may suggest that a
different dimensional structure may best explain the YGTSS.
Given that interscale correlations are as high as their reliabilities,
it is possible that all ratings are measuring the same construct and
that a single-factor may underlie the YGTSS latent structure.
Second, the 47.8-day interval used to estimate the YGTSS’s sta-
bility was relatively short; future research should examine stability
using a longer interval, particularly because slightly higher ICCs
for a subsample whose readministration interval was below the
mean may suggest lower stability over longer intervals. Third, we
did not investigate differences in YGTSS scores across various
diagnostic groups. It is uncertain the extent to which YGTSS
scores would differentiate between children with other conditions,
particularly given the high comorbidity between TS and other
disorders. Thus, it will be important to examine the ability of the
YGTSS to discriminate between children with TS and other inter-
nalizing and externalizing disorders. Fourth, some participants
were receiving pharmacological services prior to study onset.
Although no reported changes in pharmacological management
occurred during the readministration interval, prior clinical ser-
vices, changes in the effectiveness of medications, or the episodic
nature of TS (Coffey, Biederman, Geller, et al., 2000) might have
impacted the strength of the stability coefficients found. Finally,
we do not have information on the interrater reliability of the study
interviewers. On balance, one indication that rater agreement may
be adequate is the strong ICC values over time even with the use
of different raters. In other words, consistency is shown between
two YGTSS administrations separated by time and completed by
Within these positive findings, further study and refinement of
the YGTSS scoring structure may be necessary. For example, the
distinction between motor and phonic tic has good face validity,
but it is uncertain whether this scoring framework actually reflects
the true latent structure. Using exploratory factor analysis, Leck-
man et al. (1989) found a two-factor model of motor and phonic
tics that accounted for only 8% of the variance. Further study with
factor analysis will provide information regarding the format and
scoring structure of the YGTSS.
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Accepted June 17, 2005 ?
New Editor Appointed, 2007–2012
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