Among the study's weaknesses are those that follow from a natural experiment in its live setting: There is a risk of classification problems due to difficulties of ensuring internal validity in a large number of study centers.
In our study, missing item responses were more likely to occur in the school functioning scale of the PedsQL™, introducing difficulty in statistical assessment of validity and responsiveness for this domain.
The first is time, labour and expertise intensive because of the need to conceptualise a new measure and select its items, while the second is fraught with the difficulties of the relevance and validity of a measure developed in one language and culture being used in another language and culture.
EFAs aimed to select the sets of items with greater linear association with the latent score, while the IRT analyses estimated the adjust parameters and items difficulty, and both provided evidences of validity regarding internal structure.
We did not include patients whose records indicated a diagnosis of angina because of difficulty confirming the validity of this diagnosis.
These challenges include: sampling and recruitment issues, difficulties in translation and in assessment of validity with the use of standard research instruments, and increased research costs.
Another source of difficulty concerns the validity of the expert: His or her knowledge structure may diverge greatly from the knowledge structure of the actual population.
Items with an average difficulty level are those in the Range of Validity determined by the Test Information Curve.
Secondly, many of our data were analyzed retrospectively, with the inherent difficulty of ensuring the validity of the data collected.
These differences between the two groups, both the ones that are known now and the ones that may exist, but were not investigated as we wanted to keep the study simple, illustrate the main limitation of using a non-randomized design, which is the difficulty of assessing the validity of differences found, or not found, between the groups in terms of outcome of the interventions.
