The time trade-off (TTO) method is widely used to obtain quality of life scores for health states, but its validity is not uncontested, for instance because TTO values for similar health states can differ substantially between studies.
This could be caused by differences between studies in how the procedure is applied, but could also relate to differences in study samples.,
The European Journal of Health Economics
Erasmus School of Health Policy & Management (ESHPM)

van Nooten, F.E, van Busschbach, J.J, van Agthoven, M, van Exel, N.J.A, & Brouwer, W.B.F. (2018). What should we know about the person behind a TTO?. The European Journal of Health Economics, 1–5. doi:10.1007/s10198-018-0975-1