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TRA-3
PROBABILISTIC ONE-WAY SENSITIVITY ANALYSIS: A MODIFIED TORNADO DIAGRAM

**Purpose**:

Conventional tornado diagram has several challenges: it is based on deterministic one-way sensitivity analysis (1-way SA) which produces biased results when parameters are correlated. Moreover, it uses the incremental cost effectiveness ratio (ICER) which has a number of problems such as non-uniqueness. Our purpose is to generate a modified tornado diagram that addresses the above problems to improve decision-making.

**Method(s)**:

We develop a
modified tornado diagram that allows the result of probabilistic 1-way SA to be
presented clearly to decision-makers through these processes: (a) for each of
three parameters (probability, utility and cost), we fixed several (outer loop)
values. For each outer loop value, we allowed the other parameters to undergo
simulation so that when simulation is complete we observe the costs and effects
for two strategies (*21 gene assay* versus *ROR* used in genomic
tests for early-stage breast cancer) under comparison. (b)We proceeded
to calculate the incremental net monetary benefit (INMB) of *gene assay*
against *ROR*, corresponding to each outer loop value. (c) Next, we plotted
the probability density function (PDF) for each parameter with INMB on the
x-axis and probability on the y-axis; a vertical line is then drawn at the
point where the INMB is zero (decision-switching point). (d) Finally, we
calculated the *Expected Value of Perfect Parameter Information* (EVPPI) for
each parameter and arranged the PDF graphs with highest-EVPPI parameter on top
and lowest-EVPPI parameter at the bottom.

**Result(s)**:

The modified tornado diagram we have developed is shown below:

The graph shows PDF for
the two willingness-to-pay (WTP) values we used in calculating INMB. The
EVPPI for each curve and the proportion of each curve that lies above 0 on the
x-axis (thus, the probability that *gene assay* is cost-effective) are
reported on the graph. For WTP of £20,000, only the probability parameter
contributes towards decision uncertainty. For WTP of £30,000 the order of
importance of parameters to decision-makers is probability, utility, and cost.

**Conclusion(s)**:

We have executed a probabilistic 1-way SA which, while examining the sensitivity of model conclusions to changes in only one parameter's value, simultaneously takes into consideration its correlation with other parameters. We have also used INMB as model output instead of the ICER. Our approach yields a more reliable decision-making tool.

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