Background:

Older people's valuation of health-related aspects of quality of life may be altered by response shift, where they lower expectations of aspects of well-being that are believed to naturally deteriorate with age. Policy-makers may wish to adjust estimated preferences if these reflect past inequities in health funding rather than the true production possibilities. Response shift can be quantified by changing the context of the choice task. The ICECAP-O valuation exercise achieved this by asking a binary choice holistic decision of respondents, in addition to the case 2 best-worst choice task. Answers to the former are more likely to be subject to response shift since they involve traditional trade-offs. Answers to the latter reflect only 'relative disutility' of various impairments.[1]

Methods:

The scale-adjusted latent class model[2] was used to produce a tariff for each of the 313 respondents with complete classification responses and some choice data. The tariffs were used in place of the design matrix in a binary choice latent class analysis. The five attribute mean estimates from the conditional logit regressions are the attribute importance parameters of Marley et al[3] and represent the (internal) scaling factors that respondents use in transforming their case 2 BWS utilities into ones relevant in multi-profile decision-making. The principal hypothesis was that there were classes of respondents who used identical attribute importance weights. Rejection of this hypothesis prompted testing of secondary hypotheses that respondents placed lower importance on control (independence) and role (doing things that make you feel valued), those attributes thought to be most vulnerable to response shift.

Results:

17% of respondents never traded, in most cases illogically given their own ICECAP-O responses, and were dropped. Tests of parameter-equality suggested at least 30% and possibly as many as 53% of respondents for whom there is a single statistically significant attribute importance factor and 64 (21%) of respondents for whom there is a single statistically non-significant attribute importance factor. The remaining 9% of respondents had a moderate status-quo bias (preference for own life).

Conclusion and discussion:

These results do not provide strong support for response shift in the ICECAP-O valuation sample. There is only very limited support for differential weights for the five attributes when moving into a traditional DCE framework, which supports the use of reported [2, 4] case 2 BWS tariffs in allocative efficiency decisions. Low power is a limitation: the design was optimised for the BWS tasks, not the DCE. Nevertheless, this provides a framework for the quantification of response shift and adaption in future valuation exercises.

Bibliography:

1. Flynn, T.N., Using conjoint analysis and choice experiments to estimate quality adjusted life year values: issues to consider. Pharmacoeconomics, 2010. 28(9): p. 711-722.

2. Flynn, T.N., et al., Using discrete choice experiments to understand preferences for quality of life. Variance scale heterogeneity matters. Social Science & Medicine, 2010. 70: p. 1957-1965.

3. Marley, A.A.J., T.N. Flynn, and J.J. Louviere, Probabilistic Models of Set-Dependent and Attribute-Level Best-Worst Choice. Journal of Mathematical Psychology, 2008. 52: p. 281-296.

4. Coast, J., et al., Valuing the ICECAP capability index for older people. Social Science & Medicine, 2008. 67: p. 874-882.