Applies to: @RISK 6.x and Up

FAQs: Double-sided tornado graphs

 

Q1: What do the double-sided tornado graphs show?

Each bar represents a selected input and shows the range of output values when that input varies. Bars are computed by binning input values and calculating output statistics (default: Change in Output Mean) for each bin.

• Baseline: Overall simulated mean of the output.

• Bars: Edges show the minimum and maximum output means across the bins for that input.

• Calculation:

  1. Iterations are sorted by the input value.

  2. Default: 10 bins (equal number of iterations per bin).

  3. Output mean is computed for each bin.

  4. For discrete inputs, bins with identical values are pooled.

  5. Edge numbers are the min/max of the output means.

• Shading (7.5+): Shows which side of the bar corresponds to high vs. low input values, helping identify positive or negative input impact.

Note: In @RISK 6.x–7.0, the graph doesn’t show which bin corresponds to which output mean. Use correlation sensitivities or scatter plots to understand input-output trends.

Q2: Can I change the number of bins?

Yes. By default, tornado bars use 10 bins.

• To change: click the tornado icon at the bottom of the graph → Settings → "Divide input samples into ___ scenarios."

• Increasing the number of bins gives more output means with fewer iterations each.

• Avoid too many bins; each bin must have enough iterations for meaningful statistics.

Q3: How does the number of simulation iterations affect the tornado?

• More iterations → more stable output statistics.

• Fewer iterations → greater variability in output statistics.

• Variability across simulations should stay within normal statistical limits for the iteration count.

Q4: Can I use a statistic other than output mean?

Yes. You can select a Change in Output Percentile instead of Change in Output Mean.

• Computation is similar: each bin’s output percentile is calculated.

• Example: 90th percentile with 10 bins → edges = min/max of 90th percentiles per bin.

• Baseline becomes the overall output percentile.

Q5: How do I set default tornado preferences?

• Go to Utilities » Application Settings » Sensitivity Defaults.

• Set Preferred Calculation Method to Change in Output Statistic.

• You can define:

  • Number of bars

  • Statistic type (mean, percentile)

• Note: Change in Output Statistic tornado can never exceed 10 bars; for more, use correlation/regression tornado graphs or worksheet functions.

• For a specific Browse Results graph: click the tornado icon → Settings.

Attachments:
KB0248_ChangeInOutputMean.xlsx