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Investigators should justify the method chosen to combine items to create a score
or to combine domain scores to create a general score using qualitative research or
defined statistical techniques.
Total scores combining multiple domains should be supported by evidence that
the total score represents a single complex concept. Conceptual framework of a
PRO instrument: The instrument’s final conceptual framework documents the con-
cept represented by each score [ 3 ].
Validation of a PROM Instrument
Validation of different criteria of a PROM instrument requires the use of different
psychometric tests and setting off specific criteria would make the instrument ready
to be used for a certain objective fulfillment [ 54 ].
Acceptability and Data Quality—Completeness of item-level and scale-level data.
• Score distributions: a relatively low number of persons at extreme (i.e., floor/
ceiling) ends of the measurement continuum and skewness testing
• Even distribution of endorsement frequencies across response categories
(>80 %)
• Percentage of item-level missing data (<10 %)
• Percentage of computable scale scores (>50 % completed items)
• Items in scales rated “not relevant” <35 %Scaling Assumptions—Legitimacy of summing a set of items (items should mea-
sure a common underlying construct).
• Similar items’ mean scores and SDs
• Positive residual “r” between items (<0.30) to assess model prediction.
• Items have adequate corrected Item to Total Correlation (ITC ≥ 0.3).
• High negative residual “r” (>0.60) suggests redundancy.
• Items have similar ITCs.
• Items sharing common variance suggest unidimensionality.
• Items do not measure at the same point on the scale.
• Evenly spaced items spanning whole measurement range.Item Response Categories—Categories are set in a logical hierarchy.
• Ordered set of response thresholds for each scale item.Targeting—Extent to which the range of the variable measured by the scale matches
the range of that variable in the study sample.
• Scale scores spanning entire scale range
• Person-item threshold distribution: person locations should be covered by
items.M. El Gaafary