Sample size calculations in studies using the EuroQol 5D

Health-related quality of life (HRQoL) instruments are increasingly used as outcome variables in clinical trials, leading to a requirement for sample size calculations based on these variables. This paper aims to provide a gu ide to sample size calculations for use with the EuroQol-5D. The paper focu ses on sample sizes required for comparative studies, and uses scores from two reference groups of general population and critically ill patients to d etermine sample sizes using the three parts of the EQ-5D (descriptive syste m, visual analogue scale (VAS), and EQ-5D index). The effect on sample size s of different methods of categorising the three variables are compared, an d comparisons are also made between sample sizes using parametric and non-p arametric methods. Sample sizes required when the EQ-5D descriptive system is used as a binary variable (problems/no problems) are higher than or equa l to those required when each dimension is categorised in three levels of s everity (no problems, moderate problems, extreme problems). The use of thre e categories is appropriate in ill populations, though in more healthy popu lations two categories should be used. Due to the slight skewness of VAS da ta, and the equality of results using parametric and non-parametric methods , sample size calculations using the VAS should be based on a parametric ap proach. Sample sizes were considerably higher for the EQ-5D index when pred efined intervals, as opposed to a score frequency based categorisation, wer e used with the general population reference group. Using the EQ-5D index i n ill populations, it is recommended that sample size calculations are base d on parametric methods, whilst in healthier populations non-parametric met hods should be used.