RELIABILITY AND VALIDITY OF HYPOTHETICAL SCENARIOS FOR ESTIMATING WILLINGNESS TO PAY (WTP) PER QUALITY ADJUSTED LIFE YEAR (QALY) IN INDONESIA SETTING
Keywords:Reliability, validity, hypothetical scenario, Willingness-to-pay
Objective: To estimate validity and reliability of WTP questionnaire which WTP value can be taken as an indication of the monetary value of health gains, which may carry information regarding the appropriate height of the cost-effectiveness threshold.
Methods: Three hundred respondents, in Yogyakarta province, Indonesia, were interviewed during June 2017. We examine a value of WTP associated with the following scenarios: 1) improving moderate condition; 2) extending life during terminal illness, and 3) lifesaving. The interview ascertained maximum hypothetical WTP for one QALY using a dichotomous bidding format with an open-ended final question, along with questions about the socio-economic factors. Concerning validity, the WTP of the 3 versions of hypothetical scenarios were compared as known-group validity and analysis of the sensitivity and specificity was performed. Test-retest reliability and alpha Cronbach were employed to measure internal consistency.
Results: Analysis generally confirmed the validity and reliability of the WTP hypothetical scenarios. In terms of known group validity, there was significant difference across two scenarios (treatment v. s lifesaving), but no significant difference between mean WTP for treatment and terminal illness was found. Mean WTP for terminal illness (38 Million IDR) and lifesaving scenario (16 Million IDR) was significantly higher than that of treatment scenario (14 Million IDR). The WTP instrument showed good convergent validity (r=0.784), when comparing correlation between WTP value and utility score. Estimation of scenario's sensitivity and specificity in deriving expected WTP were 70.33 % and 38.98 %, respectively. The positive and negative predictive values were 64 % and 46 %. The test-retest reliability of WTP values indices excellent stability and reliability of the instrument with Spearman's rank correlation coefficient of 0.816 (p<0.001)
Conclusion: This study demonstrated that the WTP instrument is feasible and relatively reliable for measuring the WTP values in Indonesia. For wider application of the instrument, its validity should be investigated further. Meanwhile, adoption of WTP as an empirical evidence of societal values is encouraged.Â
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