Abstract:: Purpose: Despite an insidious onset without symptoms, patients with open–angle glaucoma (OAG) may experience difficulty with daily activities from. Purpose.: To investigate, using Rasch analysis, whether the item Glaucoma Quality of Life (GQL) forms a valid scale and to optimize. Abbreviation / Long Form: GQL / Glaucoma Quality of Life Questionnaire Abbreviation: GQL (>> Co-occurring Abbreviation). Long Form: Glaucoma.

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To investigate, using Rasch analysis, whether the item Glaucoma Quality of Life GQL forms a valid scale and to optimize its psychometric properties.

One hundred eighteen glaucoma patients mean age, Rasch analysis was performed to assess category function how respondents differentiated between the response optionsmeasurement precision discriminative abilityunidimensionality whether items measure a single constructtargeting whether items are of appropriate difficulty for the sampleand differential item functioning whether comparable subgroups respond differently to an individual item.

Where any of these attributes were outside acceptable ranges, steps were taken to improve the instrument. The five-response categories of the GQL were well differentiated by respondents, as demonstrated by ordered and well-spaced category thresholds. The GQL had an excellent measurement precision but demonstrated poor targeting of item difficulty to person ability and multidimensionality, indicating that it was measuring more than one construct.

Removal of six misfitting items created a nine-item unidimensional instrument with good measurement precision and no differential item functioning but poor targeting. A new name, the Glaucoma Activity Limitation GAL-9 questionnaire, is proposed for the short version, which better reflects the construct under measurement. Its only limitation is poor targeting of item difficulty to person ability, which is an inevitable attribute of a vision-related activity limitation instrument for glaucoma patients, most of whom have only peripheral visual field defects and little difficulty with daily activities.

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Nevertheless, the diagnosis, which requires lifelong follow-up and frequent ocular antihypertensive medication or surgery, can have a huge impact on a patient’s life. The patient’s point of view is measured using various types of questionnaires known as patient-reported outcomes PROs. Highlighting their importance, the US Food and Drug Administration has also endorsed that PRO measures be included in all clinical trial end points for disease impact and outcome assessment in glaucoma.

A number of glaucoma-specific questionnaires or instruments have been developed in the past two decades. Although there is no universally accepted definition of vision-related quality of life, there is growing consensus that it should include multidimensional assessment of the impact of vision on everyday activities, emotional well-being, social relationships, and independence.

Activity limitations are the difficulties in executing vision-related tasks as a result of impairment, such as inability to cross the road because of glaucomatous visual field loss. Participation restrictions are barriers to involvement in life situations caused by activity limitation, such as inability to go shopping because of inability to cross the road. Indeed, the papers describing its development explain that the purpose is to measure self-reported visual disability despite the name containing the term quality of life.

Rasch analysis estimates the raw questionnaire data to interval-level data. Interval-level data not only provide a valid measurement, they enable the use of robust parametric statistics. The primary aim of the present study was to explore the psychometric properties of the GQL using Rasch analysis and to assess whether it forms a valid scale.


If the GQL was found to form a valid scale but to have suboptimal psychometric properties, the secondary aim was to optimize its psychometric properties.

All the items in the GQL Table 1 are scored on a five-category difficulty scale, as follows: An additional category Do not perform for nonvisual reasons was scored as missing data for the final analysis. Walking after dark 3.

A comparison of the NEIVFQ25 and GQL-15 questionnaires in Nigerian glaucoma patients

Seeing at night 4. Walking on uneven ground 5. Adjusting to bright lights 6. Adjusting to dim lights 7. Going from light to dark room or vice versa 8. Tripping questionnaaire objects 9. Seeing objects coming from the questionnairw Crossing the road qurstionnaire Bumping into objects Finding dropped objects The original GQL was developed in English. The study population consisted of patients who came for regular follow-up qgl-15 the glaucoma unit of the Department of Ophthalmology at Ludwig-Maximilians-University in Munich, Germany.

The German version of the GQL was self-administered by the patients in the clinic. Patients were 18 years of age or older, spoke German, and had no cognitive impairment. Patients with all stages of glaucomatous optic nerve damage and undergoing all types of treatment, including surgery, were included. Visual acuity in the better eye had to be at least 0. Clinical assessment, including the measurement of visual acuity and visual field of each eye, was performed during the follow-up visit.

The study adhered to the tenets of the Declaration of Helsinki.

The data were analyzed in two phases: Rasch analysis is a probabilistic mathematical model that estimates item difficulty, person ability, and threshold for each response category on a single continuum logit scale. For this analysis, the person with higher ability and items of greater difficulty were located on the questionnaife side of the logit scale and vice versa.

Rasch analysis was used for the following assessments: Whether the response categories were used by respondents in the order intended was evaluated by observing whether the category calibration increased in an orderly fashion in the category probability curves a graphical display of the likelihood of each category being selected over gl-15 range of the scale.

The category threshold is the crossover point between response categories and indicates the point at which the likelihood of choosing either response category is the same.

All the items of the GQL are scored on a five-category response scale of increasing difficulty and, questionnqire, have four thresholds.

Disordering of the threshold can occur when a category is underused, its definition is unclear, or the number of categories exceeds gqo-15 number of levels respondents can distinguish. Therefore, if disordered categories are found, they should be repaired by combining adjacent categories until the thresholds are ordered.

In Rasch analysis, measurement precision of an instrument is denoted by person separation. This is a measure of discriminative ability of an instrument. A person separation of 2. The higher the person separation, the more groups the instrument can distinguish.

A fundamental element of measurement is unidimensionality; a score produced by a measure should represent a single concept. Unidimensionality is assessed by examining the fit statistics and principal component analysis PCA of the residuals.

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Both statistics indicate how well items fit the underlying construct. Infit questionnqire more sensitive to unexpected responses to items by participants whose ability is near inliers item difficulty level, whereas outfit is more sensitive to unexpected responses to items by participants whose ability is far outliers from item difficulty level. Therefore, infit is considered more informative. The standard cutoff range for MNSQ is from 0. Misfitting items should be removed to reduce noise and to optimize the psychometric properties of an instrument.


The removal of misfitting items should occur one at a time until all the remaining items demonstrate good fit. Qquestionnaire was further assessed by PCA of the residuals. The contrasts in the residuals report unexplained variance by the principal component.

Glaucoma Quality of Life Questionnaire (GQL) – Allie: Abbreviation / Long Form Info.

Similarly, items loading on first contrast by a minimum of 0. Rasch analysis generates a person-item map that provides a visual observation of the relative position of item difficulty to person ability. By default, the item mean is placed at 0 logit.

For a perfectly targeted instrument, both item and person means lie on the sample point on the map i. However, a difference of person and item means of up to 1 logit is acceptable.

Poor targeting occurs because of items clustering at a certain point along the map, large gaps between items, and the higher or lower ability of the study population than the required level of ability to endorse the items. Rasch analysis permits testing of differential item functioning DIF.

DIF occurs when different subgroups of people with comparable levels of ability respond differently to an item. Validity refers to the assessment of whether an instrument measures what it purports to measure.

Validity of the instrument was assessed by calculating correlation coefficient between the questionnaire score and traditional clinical questionnairs that is, visual acuity and visual field.

Such a validity is called criterion or external validity, which describes whether the measures of an instrument are consistent with the established measurements. Rasch analysis was performed on the GQL questionnaire data according to the Andrich Rating scale model using joint likelihood estimation with the Winsteps version 3.

Associations between normally distributed data were described by Pearson correlation coefficient, whereas Spearman rank correlation was used if one datum or both data were not distributed normally. One hundred eighteen glaucoma patients completed the German version of the GQL by self-administration.

Table 2 shows the sociodemographic and visual characteristics of the participants. On the basis of MD in better eye, Sociodemographic and Visual Characteristics of the Glaucoma Patients. The Quesrionnaire instrument demonstrated a well-functioning response scale. There were ordered qudstionnaire between all response categories, indicating that each category had distinct meaning.

Figure 1 shows the ordered response options with a distinct peak for each response category. View Original Download Slide. Category probability curves showing five well-functioning categories: The person separation value for the GQL was excellent 3.

In Figure 2the spread of each item calibration is visualized compared with the range of person ability estimates. Person-item map for auestionnaire GQL The participants are represented by x on the leftand items are located on the right of the dashed line.

The more difficult items and more visually able participants are located at the bottom of the map. Each x represents one participant. All 15 items fit the Rasch model within liberal infit 0. However, those items outside the range of 0.