Impact of the intellectual disability severity in the Spanish Personal Outcomes Scale

The interest in measuring quality of life (QoL) in persons with intellectual disability (ID) has brought about a number of QoL measurements for this population. These measurements need to address two issues that have contributed to enhancing the current instruments. First, the necessity to develop measures with adequate psychometric properties, which has been discussed in recent studies, and second, the agreement between experts in analysing objective and subjective perspectives, as well as the use of self-report to include the participation of the person with ID. The question that we set out to investigate in this paper is whether the measurements function properly for the person with ID, independent of their level of severity. We used the Spanish version of the Personal Outcomes Scale, as it is a psychometrically sound instrument and includes three sources of information (the person with ID, a professional and a family member).

The sample was composed of 529 persons with ID (296 men, representing 55.95% of the total sample, and 233 women, with M_age  = 35.03, SD = 10.82) from several regions of Spain, along with their professional of reference and a family member. The severity variable was estimated for each item based on estimations of differential item functioning.

The results showed that several items were undervalued by the assessments if the severity of the ID was greater. Mainly, this difference was observed in the assessments by professionals and in the dimensions of rights, personal development and self-determination.

This paper focuses on the uses and interpretations of the results of the QoL measurements in the Personal Outcomes Scale. The results indicate that, in our sample, when people with high levels of ID are assessed, the functioning of some items are affected by the severity of this disability. For correct use, these items must be interpreted on the basis of the results obtained. Additionally, it is necessary to thoroughly review the QoL indicators for persons with severe or profound ID.