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Catquest-9SF Patient Outcomes Questionnaire

Catquest-9SF Patient Outcomes Questionnaire - Clinical Hub, UW Health Clinical Tool Search, UW Health Clinical Tool Search, Questionnaires, Related


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ARTICLE
Rasch analyzed using a 4-Andrich rating scale model in Winsteps software. A revised version of
Catquest was developed (Catquest-9SF) and tested in 10886 patients for validity and responsive-
ness to cataract surgery.
RESULTS: Only the visual disability subscale formed a valid measurement scale. This could be en-
hanced through the addition of the 2 global assessment items; however, the symptoms and fre-
quency of performing the activities items did not contribute to the measurement. The 9-item
short-form version (Catquest-9SF) had ordered response thresholds and good person separation
(2.65) and was largely free from differential item functioning. All items fit a single overall construct
(infit range, 0.75 to 1.29; outfit range, 0.70 to 1.39) and unidimensional by principal components
analysis. The items were well targeted to the preoperative participants (0.34 logit difference in
means). The score correlated with visual acuity (r Z 0.43 preoperatively; r Z 0.48 postoperatively)
and was highly responsive to cataract surgery (preoperatively�0.32 G 2.15 logits; postoperatively
�3.21 G 2.50 logits (P<.0001).
CONCLUSIONS: The 9-item Rasch-scaled Catquest-9SF was highly valid in measuring visual
disability outcomes of cataract surgery. Its brevity makes it suited to routine clinical use,
and a raw-data to Rasch-measure conversion simplifies application.
J Cataract Refract Surg 2009; 35:504–513 Q 2009 ASCRS and ESCRS
Many visual disability questionnaires have been vali-
dated in cataract patients; these include the Activities
of Daily Vision Scale (ADVS),
1
the Visual Disability
Assessment (VDA), and the Visual Functioning 14
(VF-14)
2
. The questionnaires have been shown to be
sensitive to clinically meaningful change after cataract
surgery.
3,4
However, further validation using item-
response theory, in particular Rasch analysis, has
shown limitations in questionnaire development and
validation not previously highlighted using tradi-
tional classic test theory.
5–7
Another frequently used visual disability question-
naire is the Catquest.
8
In 1995, the National Swedish
Cataract Register began collecting data on patients’
self-assessed visual functions.
9
The Catquest was
used to measure changes in patient-reported visual
function 6 months after cataract surgery compa
with before surgery.
10
The design of the questionna
has been described.
8
Basically, questionswere asked
4 areas. The outcome was evaluated in each area
comparing a score before surgery and after surge
The total outcome of surgery was evaluated us
a decision tree, which means that the more areas t
improved after surgery, the better the outcome. Ho
ever, in each area of the Catquest, the basic princi
was to add scores achieved from patients’ choice
response options.
This summary scoring, also termed Likert scori
allocates an ordinal numerical value to a participan
response for each item. However, this method of sc
ing has limitations, not least of which is the erroneo
assumption that this method of scoring produces
Q 2009 ASCRS and ESCRS 0886-3350/09/$dsee front m
Published by Elsevier Inc. doi:10.1016/j.jcrs.2008.11
504
Catquest-9SF patient o
Nine-item short-form
of the Catques
Mats Lundstro¨m, MD, Ph
PURPOSE: To assess and optimize the Catque
outcomes of cataract surgery using Rasch analy
SETTING: Fifty-eight ophthalmic surgical units in
METHODS: Catquest questionnaires (n Z 2136
utcomes questionnaire
Rasch-scaled revision
questionnaire
, Konrad Pesudovs, PhD
t questionnaire for measuring patient-reported
is.
Sweden.
4) from the Swedish National Cataract Register

data stacked as a single data set (10478 cases but 20956
505CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
interval scale. Modern test theory, including Rasch
analysis, has shown the invalidity of summary scor-
ing
11–13
and resolves the problem by providing linear
interval transformation of the ordinal raw score,
thereby permitting the use of parametric statistical
techniques on the questionnaire data.
11,14,15
Other
unique evaluations available in Rasch analysis include
how well item difficulty targets person ability in the
population being assessed and scale validity assess-
ment, in particular item and person fit to the overall
construct.
6,13,16
As mentioned above, Rasch analysis
has been extensively used to review and improve ex-
isting questionnaires that were constructed using Lik-
ert scales.
5–7,15,17
The Rasch analysis technique has also
been used to construct new questionnaires.
18,19
In this study, we applied Rasch analysis to a data-
base of Catquest questionnaires completed before
and after cataract surgery.
7
The purpose was to assess
and reengineer the Catquest questionnaire using
Rasch analysis to optimize item fit to the construct,
minimize test length, and create a linear measure of vi-
sual disability for measuring the outcomes of cataract
surgery.
PATIENTS AND METHODS
Catquest
The Catquest questionnaire contains questions for evalu-
ating the benefit of cataract surgery.
8
There are questions
within 4 content areas: frequency of performing activities
(6 questions), perceived difficulty in performing daily-life ac-
tivities (7 questions), global questions about difficulties in
general and satisfaction with vision (2 questions), and cata-
ract symptoms (2 questions). There are 4 (summary scoring
value) response options for the perceived difficulty levels
as follows: 4 Z very great difficulty; 3 Z great difficulty; 2
Z some difficulty; 1 Z no difficulty. Therefore, a lower score
is better and a higher score is worse. The 2 items on cataract
symptoms also have these 4 response options. For satisfac-
tion with vision, the 4 response options are as follows: 4 Z
very dissatisfied; 3 Z rather dissatisfied; 2 Z fairly satisfied;
1 Z very satisfied. The frequency of performing the activity
items have 4 response options: 4 Z do not do the activity;
Submitted: September 19, 2008.
Final revision submitted: November 10, 2008.
Accepted: November 12, 2008.
From the EyeNet Sweden (Lundstro¨m), Blekinge Hospital, Karlskro-
na, Sweden, and the NH&MRC Centre for Clinical Eye Research
(Pesudovs), Flinders University and Flinders Medical Centre, Flin-
ders, Australia.
Neither author has a financial or proprietary interest in any material
or method mentioned.
Corresponding author: Mats Lundstro¨m, MD, PhD, EyeNet Sweden,
Blekinge Hospital, SE-371 85 Karlskrona, Sweden. E-mail: mats.
lundstrom@ltblekinge.se.
J CATARACT REFRACT SUR
3 Z do the activity rarely (often once a week); 2 Z do the ac-
tivity more frequently (2 to 4 times a week or for television
watching, at least 1 hour per day); 1 Z do the activity fre-
quently (every day or for television watching, several hours
per day). There are also questions about other things such as
home help, other diseases, and car driving/employment.
These latter items have been used as demographic variables,
not to evaluate the benefit of surgery, and were therefore ex-
cluded from this analysis. The items are presented in the
same format in both the preoperative and postoperative ver-
sions of the questionnaire. In this study, both preoperative
and postoperative were included to allow evaluation of the
validity of the questionnaire in both situations.
The Catquest has been used in the Swedish National Cat-
aract Register since 1995.
9
The participating surgical units
have, on a voluntary basis during 1 month each year, used
the questionnaire on all patients having cataract surgery dur-
ing that month. The number of participating units has varied
between 25 and 35. In the database, there are 23614 partici-
pants with a completed questionnaire before and after sur-
gery in the period from 1995 to 2006. The Catquest has
been used in several studies in subgroups of cataract pa-
tients.
20–23
All questionnaires were completed in Swedish;
therefore, the information about the question and response
formats presented here represents a translation.
Patients
In this study, data from 1995 to 2005 from 58 surgical units
in Sweden were used. Before the Swedish National Cataract
Register began collecting data on patients’ self-assessed
visual function, the questionnaire (Catquest) and themethod
was approved by an ethics committee according to the Dec-
laration of Helsinki and by the Swedish Data Inspection
Board. The patients were informed about the study accord-
ing to Swedish law. The data were split into 2 groups of ap-
proximately equal size. The division was made by random
selection of the Statistical Package for the Social Sciences soft-
ware (version 15.0, SPSS, Inc.). The first group was used for
the assessment and redevelopment of the Catquest (develop-
ment group). The second group was used as an independent
population to test the validity of the revised Catquest and the
outcomes of cataract surgery (validation group).
Rasch Analysis
The Catquest data were assessed for fit to the Rasch
model
24
using Winsteps software (version 3.63.2, Lina-
cre
25,26
) and the Andrich
27
version of Rasch model estimates
based on joint maximum likelihood estimation. An individ-
ual Andrich rating scale was applied for each question
format. Activity level (6 questions), perceived difficulties in
performing daily-life activities (7 questions), global ques-
tions about difficulties in general and satisfaction with vision
(2 questions), and cataract symptoms (2 questions) all have
a different format. Therefore, the analysis used a 4-Andrich
rating scale design. Each rating scale was analyzed
separately; different combinations of the scales were
included in a single analysis to determine whether a more
comprehensive overall measure was possible. The Catquest
questionnaire should be valid for measurement for both pre-
operative and postoperative patient data. Therefore, Rasch
analysis was performed on preoperative and postoperative
response sets).
28
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506 CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
Rasch analysis assumes that the probability of a respon-
dent affirming an item is a logistic function of the relative dis-
tance between the item location and the respondent location
on a linear scale. Hence, it is anticipated that the probability
of endorsing a particular category will increase monotoni-
cally with the difference between the respondent’s level of
difficulty in performing daily activities and the level of diffi-
culty required for the task. When the data meet the Rasch
model expectations, a transformation of the ordinal raw
score into a true Rasch scale is achieved.
25,27
In the case of
the Catquest, a positive person logit score suggests that the
person’s level of ability is lower than the mean required level
of difficulty for the items. Conversely, if a person logit score
is negative, the person’s perceived level of ability is higher
than the average required level of difficulty.
The key indicators of overall scale performance are person
separation and person separation reliability. These measures
are related and indicate howwell the items of the instrument
separate the respondents. Reliability ranges between 0 and 1,
with larger values indicating a greater ability to distinguish
between the strata of person ability. Separation should be
at least 2.00, and separation reliability should be at least
0.80, indicating 3 strata can be discriminated, as a minimum
level of performance to constitute a valid measure.
The presence of disordered response category thresholds
was examined before overall fit of the data to the model
was assessed. Disordered thresholds occur when partici-
pants have difficulty discriminating between ordered re-
sponse options, with categories expected to be ‘‘harder’’
being endorsed as ‘‘easier,’’ or alternatively may represent
category interchangeability rather than true disordering.
Threshold ordering is essential for the calculation of person
and item calibrations, so disordering needs to be resolved.
Interchangeable categories can be combined into a single
category to ensure ordered thresholds.
Overall fit of the data to the model was assessed using 2
overall-fit statistics: infit mean square and outfit mean
square (sum of the squared standardized residuals). Both
infit and outfit mean squares have an expected value of 1.
Values less than 0.70 represent items that overfit the model
and are too predictable; that is, they have at least 30% less
variation than expected. Conversely, mean squares greater
than 1.30 represent misfit with at least 30% more variance
than would be expected and suggests that the itemmeasures
something different than the overall scale.
29
Unidimensionality provides further evidence that the in-
strument ismeasuring the underlying trait (visual disability).
In addition to item-fit statistics, unidimensionality was as-
sessed using principal components analysis (PCA) of the
residuals and was formally tested in Winsteps by 3 indica-
tions. The first indication is comparison of the amount of
variance explained empirically and by the model; multidi-
mensionality elevates the variance explained by the model,
thus making it appear greater than the variance explained
empirically.
25
The second indication is the amount of vari-
ance explained by the first contrast (additional dimension);
while this can be tested for significance,
29
it is not appropriate
with this large dataset.
30
A threshold of 1.5 Eigenvalue is
a suitable, albeit strict, definition of multidimensionality.
31,32
The third indication is an examination of the pattern of factor
loadings on the first component to determine ‘‘subsets’’ of
items (‘‘positive’’ and ‘‘negative’’ loadings subsets).
Misfit of the data to the Rasch model could occur because
of differential item functioning (DIF), in which different
groups of the sample (ie, based on sex, comorbidity, cataract
J CATARACT REFRACT SU
status, and so forth) respond differently to individual item(s)
despite having equal levels of the underlying trait. Inspec-
tion of the raw differences in item calibration between
groups was used to identify DIF. A shift less than 0.50 logits
was considered no DIF; a shift of 0.50 to 1.0 logits, minimal
but probably inconsequential DIF; and a shift greater than
1.0 logits, at the threshold for notable DIF.
33,34
Testing for
DIF was performed between groups of sex, age (!65, 65 to
85, O85 years), with and without ocular comorbidity, first-
eye and second-eye cataract surgery, and before surgery
and after surgery.
Targeting of the items to the population was also assessed
to determine whether the questionnaire items were appro-
priate for people with cataract. Poorly targeted instruments
are often limited by floor or ceiling effects, which are dis-
played by uneven spread of items across the full range of re-
spondent’s scores and/or insufficient items to assess the full
range of the sample trait. Targeting is assessed by the pattern
of the distributions appearing on a person–item map and by
the difference in the value of the person and item mean
scores.
18
Validation Phase
Construct validation involved the testing of 2 hypotheses.
The first was that visual disability would correlate with
visual acuity. This was tested using the visual acuity data
from the registry and Pearson correlation coefficients (r).
The second hypothesis was that the visual disability score
would improve after cataract surgery. The item–category
thresholds determined during the development phase were
applied to the person responses of the second group of par-
ticipants. Overall scores were calculated for preoperative
and postoperative data to determine the impact of cataract
surgery on the Catquest score. These data were then used
to calculate the effect size (the difference between the
preoperative score and postoperative score divided by the
preoperative standard deviation).
35
RESULTS
Patients
Between 1995 and 2005, 21364 Catquest question-
naires from 58 surgical units in Sweden were com-
pleted. The mean age of the patients in the database
was 75.9 years G 9.57 (SD), and 66.1% were women.
Table 1 shows the characteristics of the patients in the
studied database. The development group comprised
10 478 patients and the validation group, 10 886
patients. There were no significantly differences be-
tween groups in any characteristic (P!.01).
Difficulty in Performing 7 Daily-Life Activities
The response scale thresholds were ordered across
the 7 items. Category probability curves for the An-
drich rating scale for these items are shown in Figure 1.
The overall fit of the data to the model seemed good,
indicating that this area formed a valid measure. The
real person separation was 2.18 (model person separa-
tion 2.41), the person separation reliability, 0.83; the in-
fit range, 0.81 to 1.24 (mean 1.0); and the outfit range,
RG - VOL 35, MARCH 2009

507CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
0.82 to 1.24 (mean 0.98). The mean of patients and
items were reasonably matched, with a mean differ-
ence of �1.18 logit (0.50 preoperatively and �2.15
postoperatively). Cronbach’s a was 0.91.
Frequency of 6 Daily-Life Activities
The frequency items did not form a valid measure.
The real person separation was 1.06 with a person sep-
aration reliability of only 0.53; therefore, this set of
items was unable to discriminate the patients. How-
ever, the response scale thresholds were disordered,
with category 3 (do the activity rarely) never being
the thresholdmost likely chosen and chiefly falling un-
der the range of category 4 (do not do the activity).
Therefore, category 4 was combined with category 3.
However, this did not improve model performance;
the real person separation was 0.92 with a person
separation reliability of only 0.46.
Cataract Symptoms
With only 2 items, the symptoms scale did not form
a valid scale. The real person separation was 0.00 with
a person separation reliability of only 0.00.
Global Assessment
With only 2 items, the global assessment scale also
did not form a valid scale. The real person separation
was 0.44 with a person separation reliability of only
Table 1. Characteristics of the 2 patient samples randomly se-
lected from the Swedish National Cataract Register.
Characteristic
Development
Group
Validation
Group
Patients (n) 10 478 10 886
Mean age (y) G SD 75.87 G 9.60 75.94 G 9.54
Sex, n (%)
Female 6948 (66.3) 7166 (65.8)
Male 3529 (33.7) 3720 (34.2)
First-eye surgery (%) 62.8 62.6
Second-eye surgery (%) 37.2 37.4
Sight-threatening ocular
comorbidity, n (%)
3478 (33.2) 3680 (33.8)
Best corrected visual acuity
Before surgery
Mean logMAR G SD 0.59 G 0.30 0.59 G 0.30
Mean Snellen 6/24 6/24
Fellow eye
Mean logMAR G SD 0.27 G 0.28 0.27 G 0.28
Mean Snellen 6/12 6/12
After surgery
Mean logMAR G SD 0.15 G 0.23 0.15 G 0.22
Mean Snellen 6/7.5 6/7.5
0.16.
J CATARACT REFRACT SUR
Combinations of Scales
If all items were included in a 4-rating scale analysis,
a valid overall scale was formed. The real patient sep-
aration was 2.46 and the patient reliability, 0.86. The
infit range was 0.55 to 1.92 and the outfit range, 0.53
to 2.41. The symptoms and frequencies items fit
poorly. If the symptoms items were retained and the
frequency items were removed, a valid overall scale
was formed; person separationwas 2.53 and reliability,
0.86. However, the symptoms items still grossly misfit
(infit range 1.84 to 2.25; outfit range 1.80 to 2.44). Re-
moval of the symptoms items and reinstatement of
the frequency items again produced a valid measure;
real patient separation was 2.48 and patient reliability,
0.86. However, the frequency items fit poorly (infit
range 0.95 to 2.00; 1.04 to 2.88). Removal of the fre-
quency items improved the overall scale performance;
real patient separation was 2.65 and patient reliability,
0.88. All items then fit a single overall construct (infit
range 0.75 to 1.29; outfit range 0.70 to 1.39). Effectively,
this means the 2 global items fit very well with the
measurement of visual disability, creating a more reli-
able measurement scale than the 7 visual disability
items alone.
Catquest-9SF
Based on the findings of the analyses combining
scales, the 7 disability items were put together with
the 2 global items to create Catquest-9SF, a 9-item
short-form (SF) measure (Table 2). The response cate-
gories were ordered (Figures 1 to 3). All items fit
a single overall construct; the infit and outfit for each
Figure 1. Category probability curves for the 7 visual disability
items.
item are shown in Table 2. Further evidence of
G - VOL 35, MARCH 2009

ti
at
r
.1
.1
.1
.1
.1
.1
.1
.1
dissatisfied; fairly satisfied; very satisfied)
.1
508 CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
unidimensionality comes from PCA analysis of the re-
siduals, which showed that variance explained by the
measures was comparable for empirical calculation
(64.2%) and by the model (64.2%). The unexplained
variance explained by the first contrast was 1.6 Eigen-
value units (6.3%), which is close to the magnitude
seen with random data. The 2 global assessment items
correlated with the first contrast (satisfaction with
vision 0.75 and overall visual difficulty 0.66), as did
the reading item (0.20); however, the magnitude of
the contrast was not enough to have much practical
impact on the person measurement. The Cronbach’s
a was 0.91. The itemswerewell targeted to the subjects
(mean difference�1.21 logits preoperatively and post-
operatively;�0.34 preoperatively and�2.32 postoper-
atively). This means that the difficulty of the items on
the questionnaire were appropriate for the ability of
patients. This is illustrated in the patient–item map
shown in Figure 4. The 2 easiest questions were recog-
nizing faces (0.95) and read text on television (0.47).
The 2 most difficult questions were needlework and
handicraft (�0.72) and satisfaction with vision (�1.19).
The preoperative Catquest-9SF scores were tested
for stability over time. Linear regression showed
a 0.05 logit reduction in score per year over the 11
years of registry data. This represents a small shift
toward patients presenting for cataract surgery at
a lower level of visual disability. This trend can be
seen in Figure 5. The change in score was significantly
different (ANOVA F
10862,10
Z 11.52; P!.001), with
post hoc testing showing that the 1995 to 1998 scores
DIF Z differential item functioning; MNSQ Z mean square; ZSTD Z standardized fit statistic
Figure 2. Category probability curves for the ‘‘satisfaction with
vision’’ item.
Table 2. The Catquest-9SF questionnaire with item difficulty calibra
preoperative versus postoperative DIF.
Item
Item Calibr
(Standard E
For the 7 difficulty items: Do you have difficulty
with the following activities because of your vision?
(yes, very great difficulties; yes, great difficulties;
yes, some difficulties; no, no difficulties)
1. Reading text in the newspaper 0.29 (0
2. Recognizing faces of people you meet 0.95 (0
3. Seeing prices of goods when shopping �0.14 (0
4. Seeing to walk on uneven ground �0.09 (0
5. Seeing to do needlework and handicraft �0.72 (0
6. Reading text on television 0.47 (0
7. Seeing to carry out a preferred hobby 0.31 (0
Two global assessment items
8. Do you experience that your present vision
gives you difficulties in any way in your daily
life? (yes, very great difficulties; yes, great
difficulties; yes, some difficulties; no, no
difficulties)
0.12 (0
9. Are you satisfied or dissatisfied with your
present vision? (very dissatisfied; rather
�1.19 (0
J CATARACT REFRACT SUR
on, infit and outfit mean square, standardized fit statistics, and
Infit Outfit
ion
ror) MNSQ ZSTD MNSQ ZSTD
DIF Preop
to Postop
) 0.81 �9.9 0.76 �9.9 0.10
) 1.29 9.9 1.28 9.9 �0.22
) 0.89 �9.9 0.93 �6.0 �0.46
) 1.25 9.9 1.39 9.9 �0.93
) 0.93 �6.1 0.89 �8.1 �0.25
) 1.05 4.3 0.99 �0.7 0.17
) 0.99 �0.7 0.87 �7.8 0.13
) 0.75 �9.9 0.70 �9.9 0.56
) 1.03 2.4 1.04 3.5 1.02
G - VOL 35, MARCH 2009

509CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
were significantly different from the 2001 to 2005
scores.
The Catquest-9SF was largely free of DIF. Two items
showed a small level of DIF by sex; seeing to walk on
uneven ground (0.64 logits) and seeing to do needle-
work and handicraft (0.62 logits) were both rated by
women as easier relative to the other tasks. Four items
showed a small level of DIF by age as follows: seeing to
walk on uneven ground (rated 0.53 logits relatively
easier in the 65- to 84-year group than in the !65-
year group and rated 0.89 logits relatively easier in
the O85-year group than in the !65 years group);
seeing to do needlework and handicraft (0.56 logits
relatively easier in the O85-year group than in the
!65-year group); global assessment of difficulties
(0.57 logits relatively more difficult in the 65- to 84-
year group than in the !65-year group and 0.72 logits
relatively more difficult in the O85-year group than in
the !65-year group); and global assessment of satis-
faction with vision (0.54 logits relatively more difficult
in the O85-year group than in the !65-year group).
There was neither DIF by the presence or absence of
comorbidity nor by first-eye or second-eye cataract
surgery status. For an instrument to be used on preop-
erative and postoperative populations, it is important
that item functioning is consistent acrossmeasurement
time frames. Therefore, DIFwas tested between preop-
erative and postoperative data sets. Three items
showed some DIF: seeing to walk on uneven ground
(rated 0.93 logits relatively easier in the postoperative
ranking); global assessment of difficulties (rated 0.56
logits relatively more difficult in the postoperative
ranking); and global assessment of satisfaction with
Figure 3. Category probability curves for the ‘‘difficulty in perform-
ing daily-life activities in general’’ item.
J CATARACT REFRACT SU
vision (rated 1.02 logits relatively more difficult in
the postoperative ranking).
The Rasch analysis of the Catquest-9SF not only pro-
vided the item calibrations found in Table 2 but also
item–category calibrations for each of the 4 response
categories of the 9 items. These 36-item–category cali-
brations can be used as anchor values to convert ordi-
nal category value to Rasch measurement estimates.
This is valid for both preoperative and postoperative
questionnaire data because the calibrations were de-
veloped using a combination of preoperative and post-
operative data. Other investigators wishing to use the
Catquest-9SF can use these calibrations to achieve
Rasch measurement without the need to perform
Rasch analysis. An Excel spreadsheet has been created
for this purpose and is available from the authors.
Validation Phase
The preoperative and postoperative Catquest data
from the second population (n Z 10 886) were
Figure 4. Person–item map of the 9-item short-form Catquest for the
preoperative development group showing the distribution of Rasch-
calibrated participant scores (left) and item locations (right). The
items are well targeted to the patients as illustrated by the matching
of the distributions (M Z mean, S Z 1 standard deviation, T Z 2
standard deviations.
RG - VOL 35, MARCH 2009

510 CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
converted to Rasch person estimated using the item–
category anchor calibrations established in the devel-
opment phase. These were correlated against preoper-
ative and postoperative visual acuity data. The
preoperativeCatquest-9SF score correlatedwith visual
acuity in the eye to be operated on (r Z 0.207), visual
acuity in the fellow eye (r Z 0.410), and visual acuity
in the better eye (r Z0.431). The postoperative Cat-
quest-9SF score correlated with visual acuity in the op-
erated eye (r Z 0.443), visual acuity in the fellow eye
(rZ0.363), andvisual acuity in thebetter eye (rZ0.476).
ThemeanpreoperativeCatquest-9SF scorewas�0.32
G 2.15 logits. The mean postoperative Catquest-9SF
scorewas�3.21G 2.50 logits. This 3-logit improvement
was statistically significant (P!.0001, paired 2-tailed
t test). Figure 6 shows a scatterplot of the preoperative
and postoperative Catquest-9SF scores. This shows
that the majority of cases improved, as illustrated by
the bulk of the data appearing below the 1:1 line; 9.8%
of cases had a poorer Catquest-9SF score after cataract
surgery than before surgery. The change in score with
cataract surgery represents an effect size of 1.35.
DISCUSSION
The application of Rasch analysis to an existing con-
ventionally developed questionnaire provides 2 dis-
tinct benefits. First, a greater insight into internal
consistency is provided through the fit of the items to
the model. In the case of Catquest, it was shown that
the visual disability items were internally consistent
and that the measurement of visual disability could
Figure 5. Box plot showing a slight trend toward lower levels of pre-
senting visual disability over the decade of Swedish National Cata-
ract Register data (boxes represent the middle half of the ranked
data; black lines are the median values; bars indicate 95% confidence
intervals).
be augmented through the addition of the 2 global
J CATARACT REFRACT SUR
assessment items because thesewere conceptually con-
sistent with the same underlying trait. However, nei-
ther the symptoms items nor the frequency items
were consistent with this trait, as illustrated by misfit
to the model. This finding is consistent with research
of quality-of-life instruments in which symptoms and
disability failed to tap the same latent trait.
18,36
As de-
scribed previously, the evaluation of original Catquest
was made using a decision tree. The most important
part was the 7 disability items. The Rasch analysis
has confirmed that this part of the questionnaire repre-
sented the most valid measurement, which supports
the role of these items in the original questionnaire.
The refinement of the scale to purely measure visual
disability is consistent with the World Health Organi-
zation International Classification of Functioning, Dis-
ability and Health definitions.
37
The second benefit of Rasch analysis is the scoring of
patient ability on a valid interval scale. This improves
the precision of measurement by eliminating noise
from nonlinearities in the summary scoring. Clearly,
this results in more meaningful interpretation of scor-
ing and also reduces the sample size required to find
significant differences in clinical outcome studies.
14,15
The finding that the frequency items did not contrib-
ute to themeasurement of disabilitywas not consistent
with the theory behind the inclusion of these items in
the original Catquest. It was expected that how often
an activity was performed would give an indication
of the importance of the activity and would therefore
modulate visual disability. This expectation was not
met by finding that including frequency of performing
an activity in the same scale as visual disability actually
Figure 6. Scatterplot of preoperative Catquest-9SF score (logits) ver-
sus postoperative Catquest-9SF score (logits). The majority of cases
improved, as shown by the points appearing below the 1:1 line.
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511CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
adds noise to the measurement. Perhaps this indicates
that people naturally take into account how often they
perform an activity or how important an activity is to
them when they assess how much difficulty they are
having performing the activity.
The combination of perceived disabilities in daily-
life and global assessment items formed the abridged
version of Catquest with the greatest measurement
precision. Therefore, we propose that the 9-item short
form of the Catquest (Catquest-9SF) should be used in
place of the original version. The Catquest-9SF has
excellent precision and is unidimensional, as indi-
cated by the fit statistics and PCA of the residuals.
In addition to the questionnaire’s internal consis-
tency, the use of only 9 items greatly reduces the re-
spondent burden, which makes the questionnaire
suitable for use in clinical practice. Developing short-
er questionnaires for broader implementation of
patient-reported outcomes measurements is an
often-pursued aim.
6,38,39
However, Rasch analysis
has shown limitations in these questionnaires. It is
difficult to have satisfactory measurement precision
with a small number of items.
6
Attempts to create
short-form questionnaires from existing question-
naires has arrived at minimum item sets of 12 items
for the VDA, 16 items for the ADVS, and 10 items
for the VF-14 (Pesudovs K, et al. IOVS 2005;
45:ARVO E-Abstract 3844).
5,7
However, both the
ADVS and VF-14 showed poor targeting of item dif-
ficulty to patient ability, with the VF-10 requiring ad-
ditional new items to optimize measurement. Not
only can the Catquest-9SF measure with satisfactory
precision, the items are well targeted to patient abil-
ity. Good targeting of a short-form visual disability
instrument also occurs with the Cataract Outcomes
Questionnaire (Pesudovs K, et al. IOVS 2005;
45:ARVO E-Abstract 3844). The better functioning of
these short-form instruments compared with that of
the ADVS and VF-14 is likely because the content of
the long-form instruments is better targeted to patient
ability, thus enabling a short form to be created while
maintaining good targeting. Targeting is population
dependent, and these data prove only that the Cat-
quest-9SF is well targeted to the Swedish cataract
population. Targeting should be tested for other pop-
ulations, although the extremely large patient pool
used in this study makes it unlikely that this is an
atypical population for a developed country. The
main issue for international adaptation is variation
in the indications for cataract surgery. A reduction
in the threshold visual impairment or disability for
cataract surgery has been widely reported.
40
Although there has been a small shift toward patients
presenting for cataract surgery at lower levels of
visual disability over the 11 years of the data used
J CATARACT REFRACT SUR
in this study, the change is not large enough to ad-
versely affect targeting of item difficulty to patient
ability. Also, although optimal targeting occurs for
the preoperative population, in postoperative cases,
the items are too easy for the patients. This is an ac-
ceptable compromise in a cataract-related visual dis-
ability instrument because the goal of cataract
surgery is to eliminate visual disability; therefore,
by definition, targeting will change. This can lead to
a ceiling effect. Figure 6 shows that some postopera-
tive score are at the ceiling; thus, there must be
some measurement distortion for these individuals.
However, many postoperative patients are well mea-
sured without distortion. That the Catquest-9SF can
still discriminate postoperative patients with good
precision is an important indicator of its validity for
clinical measurement of cataract outcomes.
Although the large sample size in this study has sev-
eral advantages, it placed several limitations on the
analyses. Significance-based testing was not possible
because the extreme sample size provided so much
power that any level of misfit would be identified as
significant.
30
Nevertheless, by using magnitude as
the indicator of error, the Catquest-9SF was shown
to be largely free of DIF and to be unidimensional.
The Rasch scaling of the Catquest-9SF provides for
a spreadsheet-based conversion of raw data to Rasch
estimates. Therefore, clinicians who wish to measure
cataract surgery outcomes using Catquest-9SF do not
have to apply Rasch analysis to obtain the benefits of
true interval data, such as suitability for parametric
statistical analysis. The development of this conver-
sion algorithm in such a large population suggests
that the model will be very stable. This method is
robust to even large amounts of missing data.
The Catquest-9SF was shown to correlate well with
visual acuity in the better eye. This is consistent with
findings in other studies that show that better-eye vi-
sual acuity correlates best with visual disability mea-
surement.
41
Moreover, the correlations with visual
acuity here are higher than for most questionnaires,
which is likely due to the high precision of the Cat-
quest-9SF.
42,43
The Catquest-9SF was highly respon-
sive to cataract surgery. The effect size reported here
can be considered to be large; convention holds that ef-
fect sizes of 0.20 to 0.49 are considered small, 0.50 to
0.79 are moderate, and 0.80 or above are large.
44
The good characteristics of Catquest-9SF have led to
its incorporation in the SwedishNationalCataract Reg-
ister. Future work using the Catquest-9SF will include
a more comprehensive evaluation of the outcomes of
cataract surgery. The relative benefit of first-eye and
second-eye cataract surgery and the role of cofactors
such as ocular comorbidity, age, sex, and location
will be reported in a subsequent manuscript.
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512 CATQUEST-9SF PATIENT OUTCOMES QUESTIONNAIRE
The importance of Rasch analysis in the develop-
ment and scoring of questionnaires has been recog-
nized in standards proposed for the assessment of
questionnaire quality.
16,45,46
Numerous calls have
been made in ophthalmology for the development of
instruments using this technology.
11,47,48
Clearly,
a Rasch-scaled questionnaire should be usedwherever
possible for the measurement of outcomes.
48,49
The
Catquest-9SF, along with the Cataract Outcomes
Questionnaire, represent the state of the art for the
measurement of visual disability, and is likely superior
to the Rasch-analyzed versions of the ADVS and the
VF-14 and certainly superior to any visual disability
instruments not subjected to Rasch analysis (Pesudovs
K, et al. IOVS 2005; 45:ARVO E-Abstract 3844).
In conclusion, the Catquest-9SF is a valid short-form
visual disability instrument that is ideal for the mea-
surement of patient-reported outcomes of cataract sur-
gery. From a clinical viewpoint, the advantages with
this instrument include that it measures disability,
gives interval scale scoring, has high precision, is short
and minimizes response burden, is sensitive to
changes after cataract surgery, has high effect size,
and has good targeting.
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First author:
Mats Lundstro¨m, MD, PhD
EyeNet Sweden, Blekinge Hospital,
Karlskrona, Sweden
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