Purpose
Tornado diagrams, also called tornado plots or tornado charts, are a special type of Bar chart, where the data categories are listed vertically instead of the standard horizontal presentation, and the categories are ordered so that the largest bar appears at the top of the chart, the second largest appears second from the top, and so on. They are so named because the final chart appears to be either one half of or a complete tornado.
Tornado diagrams are useful for deterministic sensitivity analysis - comparing the relative importance of variables. For each variable/uncertainty considered, you will need estimates for what the low-base-high outcomes would be. The sensitive variable is modeled as uncertain value while all other variables are held at baseline values (stable). - Reference/source: PMBOK Guide Fifth Edition (2013) pg. 338 (4th Ed.,2008, pg. 298). This allows us to test the sensitivity/risk associated with one uncertainty/variable. For example, if a decision maker needs to visually compare 100 budgetary items, and wishes to identify the ten items (s)he should focus on, it would be nearly impossible to do using a standard bar graph. In a tornado diagram of the budget items, the top ten bars would represent the items that contribute the most to the variability of the outcome, and therefore what the decision maker should focus on.
Read more about this topic: Tornado Diagram
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