Two ways to skin a cat: a comparison of two variants of standard gamble

1. Two ways to skin a cat: a comparison of two variants of standard gamble John Brazier and Paul Dolan Prepared for the CHEBS workshop on Elicitation, 9 October 2002

2. full health p dead 1-p state Hi Standard Gamble:Chronic Health States where full health  Hi  death vary ‘p’ until the respondent is indifferent between the alternatives

3. Variants of SG The ‘ping pong’ variant (PPV): interview administered with the aid of props using a ‘ping pong’ procedure The ‘titration’ variant (TV): self-completed; presents a list of probability of success values (from 100 in 100 down to 0 in 100) for ticking No theoretical arguments for choosing between them

4. Please put a Ö against all cases where you are CONFIDENT that you would CHOOSE the risky treatment (Choice B). Please put an X against all cases where you are CONFIDENT that you would REJECT the treatment (Choice B) and accept the certain health state (Choice A). Please put an = against all cases where you think it would be most difficult to choose between the treatment (Choice B) and accept the certain health state (Choice A). Chances of success Chances of failure 100 in 100* 0 in 100* 95 in 100* 5 in 100 90 in 100 10 in 100 85 in 100 15 in 100 80 in 100 20 in 100 75 in 100 25 in 100 70 in 100 30 in 100 65 in 100 35 in 100 60 in 100 40 in 100 55 in 100 45 in 100 50 in 100 50 in 100 45 in 100 55 in 100 40 in 100 60 in 100 35 in 100 65 in 100 30 in 100 70 in 100 25 in 100 75 in 100 20 in 100 80 in 100 15 in 100 85 in 100 10 in 100 90 in 100 5 in 100 95 in 100 0 in 100 100 in 100

5. Study questions • Do the two variants produce the same values? • If they produce different values, then why?

6. Design • Convenience sample of staff and students at UoS • Randomised into two groups: G1 and G2 • Both groups completed the SF-6D descriptive system and then ranked and valued seven states defined by the SF-6D • G1 valued the first 3 states plus PITS by PPV and the remaining 3 states by TV • G2 valued the first 3 states plus PITS by TV and the remaining 3 states by PPV

7. Example of (SF-6D) health states Your health limits you a lot in moderate activities (such as moving a table pushing a vacuum cleaner, bowling or playing golf) You are limited in the kind of work or other activities as a result of your physical health Your health limits your social activities (like visiting friends, relatives etc.) most of the time You have pain that interferes with your normal work (both outside the home and housework) moderately You feel tense or downhearted and low most of the time. You have a lot of energya little of the time

8. Design (2) • The six states paired where each pair differed only in one level of one dimension • For six states, full health and PITS (i.e. the worst state defined by the SF-6D) were the reference states and PITs was then valued against full health and death • Both groups valued states in the same order • At the end of the interview, respondents asked about difficulty and understanding and the reasons for any apparent inconsistencies

9. Results (1) • G1 (n=28) and G2 (n=30) comparable in terms of gender, age and own health • Rankings similar and VAS ratings of states virtually identical • Mean values generated by TV higher than PPV for first four states (by .107, 0.38, .13 and .138), but little or no difference for the second set of states • TV had a higher proportion of 1.0 responses than PPV (25 vs. 6)

10. Mean SG health state values

11. Results (2) Logical ordering (Z>X, Q>Y, T>R) • VAS values consistent • Inconsistency between variant: G1 Z(PPV) vs. X (TV) and G2 Q(TV) vs. Y(PPV) • Inconsistency within variant: PPV - Z and X and TV – Q and Y

12. Respondents comments • Inconsistencies due to confusion/forgetting; some found the PPV made gambling more attractive (‘yo’s yo’s’ around) • Understanding and degree of difficulty favoured PPV • Found PPV easier and liked the visual aid • A small number commented that they preferred TV since PPV was confusing

13. Discussion (1) • TV produced higher values than PPV • This Difference only existed for the 4 states valued first by each group – this represents a ‘pure’ variant effect • Second set of states also influenced by what went before – possible reference point effect whereby TV valuations in G1 dragged down by respondent memory of PPV valuation of first 4 states (and vice versa) – which offset variant effect

14. Explanations for results Anchoring effect: TV starts respondents at upper end whereas PPV iterates respondents between upper and lower end Confusion effect: the ping pong of PPV confuses respondents e.g. being iterated from 100% chance of success to 10% for mild states Heuristics: respondents may be using cognitive strategies or shortcuts that operate to simplify cognitive functioning requirements of these tasks

15. Implications • No basis for choosing one variant over the other • More research into the way people answer these questions and what influences their response • Re-design tasks by combining different procedures, props and methods of administration (e.g. TV with props) • Supports the idea that preferences are not merely articulated in these exercises, but are being constructed