Measuring Life Satisfaction in Parkinson's Disease and Healthy Controls Using the Satisfaction With Life Scale.

The 5-item Satisfaction With Life Scale (SWLS) was designed to measure general life satisfaction (LS). Here we examined the psychometric properties of the SWLS in a cohort of persons with Parkinson`s disease (PwPD) and age and gender matched individuals without PD. The SWLS was administered to PwPD and controls from the Norwegian ParkWest study at 5 and 7 years after the time of diagnosis. Data were analysed according to classical test theory (CTT) and Rasch measurement theory. CTT scaling assumptions for computation of a SWLS total score were met (corrected item-total correlations >0.58). The SWLS was reasonably well targeted to the sample and had good reliability (ordinal alpha, 0.92). The scale exhibited good fit to the Rasch model and successfully separated between 5 statistically distinct strata of people (levels of SWLS). The seven response categories did not work as intended and the scale may benefit from reduction to five response categories. There was no clinically significant differential item functioning. Separate analyses in PwPD and controls yielded very similar results to those from the pooled analysis. This study supports the SWLS as a valid instrument for measuring LS in PD and controls. However, Rasch analyses provided new insights into the performance and validity of the SWLS and identified areas for future revisions in order to further improve the scale.