COMPARISON OF MATHEMATICAL AND BEHAVIOURAL ELICITATION APPROACHES: APPLICATION USING A DECISION MODEL FOR TOPICAL NEGATIVE PRESSURE (TNP) THERAPY FOR PRESSURE ULCERS

Purpose
There is little guidance on the conduct of elicitation to inform decision models. We explored two approaches: implicit synthesis using behavioural aggregation and explicit synthesis using mathematical elicitation. In addition the performance of experts and elicitation approaches was assessed using calibration methods.

 Methods
The elicitation exercise was designed to generate initial estimates for a model investigating topical negative pressure (TNP) therapy for pressure ulcers. Distributions were elicited for six parameters from seven experts using the histogram approach. Four parameters, concerning the effectiveness of chronic wounds treatments, had robust known values and were used to calibrate experts in the mathematical approach. The remaining two questions were used to generate priors on the impact of TNP therapy

 For the behavioural method the same seven experts repeated the elicitation exercise but using the nominal group approach. In the mathematical approach, histograms were synthesised using linear pooling. At the end of the elicitation experts were asked for their views on the two approaches.

 Results
There was some variation in elicited values between individuals and between elicitation approaches. The behavioural method produced irrational responses to two known parameters, suggesting a shorter healing time for 70% of patients compared to the median healing time.

 Values for the two unknown parameters were higher in the behavioural approach compared with the mathematical approach. Behavioural values suggest that almost 17% more patients will not have healed 6 months after treatment with TNP. In this study calibration appeared to have little impact on estimates of the unknown parameters.

 There were mixed messages from experts regarding the ease of the two approaches.

 Conclusions
There was no clear appropriate approach in this small study. Given the ambiguity of experts’ views regarding the two approaches, the choice must be governed by the extent to which it is fit for purpose, at least from a theoretical perspective. The focus on achieving consensus means that behavioural approaches do not consider the inherent uncertainty in expert’s beliefs about a parameter. Any decision model including data elicited from experts in this manor will not fully characterise the uncertainty in the adoption decision. However the choice of synthesis methods provides an additional level of uncertainty with the mathematical approach.  
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