OBJECTIVES: The Quality of life in Depression Scale (QLDS) is the most widely used measure of quality of life (QoL) in clinical trials and studies of depression. The instrument has been validated in 18 languages. The instrument, which adopts the needs model, is highly sensitive to changes in QoL and is well accepted by respondents. Since its development new statistical models have been introduced into instrument development. The present study was designed to adapt the QLDS using item response theory in order to improve its measurement properties further.
METHODS: Existing datasets were subjected to Rasch analysis to identify misfitting items and to look for gaps in the range of measurement. As weaknesses were identified at the extremes (very good or very poor QoL) additional interviews were conducted with patients scoring either high or low on the QLDS and new qualitative analyses were made of the 40 original interview transcripts. Potential new items were added to the QLDS and poorly performing ones removed. This version was administered to samples of depressed patients in the UK, US, France and Germany. The data were again subjected to Rasch analysis and a final version identified. Its psychometric properties were then assessed in a second postal survey.
RESULTS: A 30-item version was identified with little item misfit or differential item functioning in the different countries. This version had excellent psychometric properties including reproducibility and construct validity. The main gain was an increase in the measurement range from 2.5 to 4.3 logits, an increase of 72%.
CONCLUSIONS: The measurement properties of the QLDS have been improved, ensuring it provides valid scores on a unidimensional QoL scale. The increased measurement range makes the instrument more suitable for use across the full range of depression severity and hence more valid for use in antidepressant maintenance studies.
