Tornado Diagram
In economic evaluations, tornado diagrams are used to visually present the results of multiple invert sensitivity analyses on a single graph. This graphical representation helps to identify which parameters have the most significant impact on the model’s output, typically an incremental cost-effectiveness ratio (ICER).
Key aspects of a tornado diagram include:
– Univariate Sensitivity Analysis: Each bar in the diagram represents the variation in the model output when a single parameter is varied between two plausible but extreme values, while all other parameters are held constant at their base case values.
– Horizontal Bars: Each sensitivity analysis is summarized using a horizontal bar. The length of the bar represents the range of the model output (e.g., ICER) resulting from the variation in the parameter.
– Ordering by Sensitivity: The horizontal bars are ordered from top to bottom based on the magnitude of their spread. Parameters with the greatest impact on the model output (i.e., those causing the most significant variation in the ICER) are placed at the top of the diagram, while those with the least impact are placed at the bottom.
– Distinctive Shape: The resulting stacked horizontal bars create a distinctive shape resembling a tornado, hence the name “tornado diagram.”
– Purpose: Tornado diagrams are used to help reviewers and decision-makers quickly assess which parameters have the greatest influence on the results of the economic evaluation. This visual tool highlights the robustness of the model and identifies key areas where additional data or research may be needed to reduce uncertainty.
Example:
Imagine an economic model evaluating the cost-effectiveness of a new drug. The model’s base case ICER is $50,000 per QALY. A tornado diagram is constructed by varying each parameter (e.g., drug cost, treatment efficacy, adverse event rate) between plausible extremes and observing the impact on the ICER. The parameter causing the ICER to range from $40,000 to $60,000 (a $20,000 spread) would have a longer bar than a parameter causing a range from $48,000 to $52,000 (a $4,000 spread). The bars are then ordered by these spreads, with the most influential parameters at the top.
Usefulness:
Tornado diagrams are valuable tools in economic evaluations for visualizing the sensitivity of the model to changes in input parameters. They provide a clear and concise way to understand the relative importance of different parameters and to prioritize areas for further investigation or data collection. By identifying the parameters that most influence the results, decision-makers can better understand the robustness of the model and focus on reducing uncertainty in those critical areas.
Click HEOR TERMINOLOGY