Enhancing precision of the 16-item Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE-16) using Rasch methodology

Development of a self-report measure of cognitive change: assessment of interpretability in two samples, people with HIV and people without HIV

AIM

The overall aim of this study was to develop a method of measuring change in cognitive ability from the person's perspective.

METHODS

Cognitive change items came from an item pool that was used to develop the Communicating Cognitive Concerns Questionnaire (C3Q). The change items were administered to a test sample of 211 people with HIV + and a sample of 484 people drawn from the general population (HIV- sample). Rasch analysis was used to identify items that formed a linear continuum and correlations with measures of related constructs were used to support the interpretability of the new measure.

RESULTS

Eleven of the original 12 change items fit the unidimensional Rasch model in both samples with a near similar ordering of the items. The average value for cognitive change of the HIV + sample was greater than that of the HIV- sample. Values on C3Q-Change correlated most highly (> 0.7) with current self-reported cognitive status and measures of depression and anxiety (> 0.6). The lowest correlation was with performance-based cognitive ability (r = 0.2).

CONCLUSION

The items of C3Q-Change fit a strong measurement model and related to converging constructs in an expected way. Further work needs to be done to assess the meaning of self-reported cognitive change in relationship to measured change and to examine sources of differential item functioning.

Establishing conversion of the 16-item Informant Questionnaire on Cognitive Decline in the Elderly scores into interval-level data across multiple samples using Rasch methodology

BACKGROUND

The 16-item Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE-16) is a well-validated and widely-used measure of cognitive changes (CCs) among older adults. This study aimed to use Rasch methodology to establish psychometric properties of the IQCODE-16 and validate the existing ordinal-to-interval transformation algorithms across multiple large samples.

METHODS

A Partial Credit Rasch model was employed to examine psychometric properties of the IQCODE-16 using data (n = 918) from two longitudinal studies of participants aged 57-99 years: the Older Australian Twins Study (n = 450) and the Canberra Longitudinal Study (n = 468), and reusing the Sydney Memory and Ageing Study (MAS) sample (n = 400).

RESULTS

Initial analyses indicated good reliability for the IQCODE-16 (Person Separation Index range: 0.82-0.90). However, local dependency was identified between items, with several items showing misfit to the model. Replicating the existing Rasch solution could not reproduce the best Rasch model fit for all samples. Combining locally dependent items into three testlets resolved all misfit and local dependency issues and resulted in the best Rasch model fit for all samples with evidence of unidimensionality, strong reliability, and invariance across person factors. Accordingly, new ordinal-to-interval transformation algorithms were produced to convert the IQCODE-16 ordinal scores into interval data to improve the accuracy of its scores.

CONCLUSIONS

The findings of this study support the reliability and validity of the IQCODE-16 in measuring CCs among older adults. New ordinal-to-interval conversion tables generated using samples from multiple independent datasets are more generalizable and can be used to enhance the precision of the IQCODE-16 without changing its original format. An easy-to-use converter has been made available for clinical and research use.