Validation of the Catquest-9SF Questionnaire in Greek Language

Rasch Analysis: Person Separation Index (PSI)

PSI contributes to the assessment of the measurement precision of the Greek Catquest-9SF. PSI implies the questionnaire's ability to differentiate along its scale. The greater the value of person separation, the greater the questionnaire precision (acceptable person separation \> 3.0).

3 months

Rasch Analysis: Person Reliability (PR)

PR contributes to the assessment of the measurement precision of the Greek Catquest-9SF. PR (ranging from 0 to 1) should be \> 0.9 to be considered adequate, indicating a high ability to distinguish between the strata of person ability.

3 months

Rasch Analysis: Differential Item Functioning (DIF)

DIF indicates the differences between different subgroups of patients, since the questionnaire item calibration should be comparable across different patient groups. DIF occurs if patients from different subgroups with the same underlying true ability have an unequal probability of giving of a response to an item. DIF is assessed by evaluating age, gender, existence of ocular comorbidities, first vs second eye cataract surgery and binocular distance BSCVA. DIF of \>1.0 logits is significant, DIF with values between 0.5 and 1.0 is minimal, and DIF \<0.5 logits is considered absent.

3 months

Rasch Analysis: Τargeting

Τargeting of the questions to the patient sample, which indicates how well the items' difficulties match the person ability, is assessed to establish if the questions are appropriate for patients before and following cataract surgery. This can be assessed graphically by observing the spread along the person-item map and by the difference between the item and the person mean values. A well-targeted instrument requires the ability of the patients and the difficulty of the questions to be centered on the same mean (targeting of 0). In general, a mean difference in magnitude of ≥1 logit indicates significant mistargeting, while a difference of \<1.0 logit indicates good targeting, with targeting of 0 being ideal. A positive value indicates that the items are too difficult relative to person's ability, while a negative value indicates that the items are too easy. A person-item map also visualizes the item hierarchy of difficulty, ranging from a minimum to maximum difficulty of performance.

3 months