Sorry, Mahdi, if I didn't make myself clear on that, and maybe confused you about tornado vs. influence diagram.
I am very familiar with sensitivity analysis (besides its use for risk analysis), and it is a method you can use to quantify how sensitive a given parameter is to variations in a number of factors, typically affected by uncertainty. So, this technique helps you quantify the impact of these uncertain factors on the parameter of your interest. When it comes to risk analysis, you may want to use this method to assess the impact of risk factors over one or more parameters (e.g., the project duration). One way to represent the results obtained through sensitivity analysis is through tornado diagrams; see Figure 11-14 from the PMBOK Guide as an example. In that case, you performed sensitivity analysis to quantify the impact of a number of risks on project duration. A positive correlation implies that each factor on the y-axes determines an increase in project duration; a negative correlation has the opposite effect. The higher the impact, the bigger the correlation.
I am not as much familiar with influence diagrams, but I wouldn't say they both produce the same result. The way I see it, is that they provide you with different information, in that influence diagrams visually depict the causal relationships between factors (e.g., A influence B and is influenced by C), possibly showing the time sequence they follow, and some of these factors - or the outcomes they produce - can be uncertain. These uncertain factors can be characterized by a probability of occurrence and/or a range of variation. Once you identify these relationships, you may want to use other techniques (including sensitivity analysis) to identify the factors having the highest impact on the parameter of interest for you.
Tornado diagrams are very different from influence diagrams in the way they represent information, so I wouldn't say tornado diagrams fall under the influence diagram category. In fact, while tornado diagram are a special type of bar chart, influence diagram resemble more to flowcharts.
Hope this clarifies a bit what I meant in my previous reply.
I'll be happy to hear from others being more experienced than me with influence diagrams, particularly their use in risk analysis.
Gabriella
