# Base Case Tornado Diagrams

The Base Case Tornado Diagram evaluates the outcome of a model when chance events are individually set to high and low states and compares the results to the outcome of the model when all chance events are set to a nominal state. The Initial Decision Alternatives Tornado Diagram is essentially two or more Base Case Tornado Diagrams on one chart: one tornado for each alternative of the initial decision.

Initial Decision Alternatives Tornado Diagrams are useful when you want to compare the sensitivity of your chance events across your initial decision alternatives.

__Generating a Base Case Tornado Diagram__

- Select Home | Sensitivity | Tornado | Base Case. The Tornado Setup dialog will come up.
- Assign Low, Nominal and High states for the chance events in the model (see "Assigning States" below).
- Click OK.

__Generating an Initial Decision Alternatives Tornado Diagram__

- Select Home | Sensitivity | Tornado | Initial Decision Alternatives. The Tornado Setup dialog will come up.
- Assign Low, Nominal and High states for the chance events in the model (see "Assigning States" below).
- Click OK.

The process for setting up these two types of tornado diagrams is identical, so the instructions below apply to both types of tornado diagram. Only one setup process is required for an Initial Decision Alternatives Tornado Diagram because the assignment of states must be the same for each initial decision alternative.

__Assigning States__

To run this analysis, you need to specify for each event which state should be considered the low state, the nominal state and the high state. When the dialog first appears, DPL assigns the first state to be the low state and the last state defined to be the high state. DPL assigns the nominal state to be the default state (as specified in the node definition) unless the default state is undefined or equal to the first or last state. If the default state is undefined or is equal to the first or last state, then DPL assigns the state nearest the middle to be the nominal state. (If there are an even number of states, DPL chooses the first of the two middle states.)

When you run a Base Case or Initial Decision Alternatives Tornado Diagram, the Tornado Setup dialog presents a list of all the uncertain events in the model. Choose OK to run the analysis using the assignments DPL has made otherwise use the drop down lists in the table to change the low, nominal and high assignments as needed.

NOTE: if you have a chance event with three or more states and you specify the same state for any two of the low, nominal and high assignments, DPL will warn you. Also note that if you have a two-state chance event, then one of the states will have to be repeated for one the low, nominal or high assignments.

__Results and Policy Changes__

To produce a Base Case Tornado Diagram DPL will run the model several times: once with all chance events set to their specified Nominal states to establish a base case policy and outcome value. And then twice for each chance event, setting it once to its specified Low state and once to its specified High state while all other events are set at their specified Nominal states. It will then display a bar for each event indicating the difference in the output value under the optimal policy as the event is varied from the Low to High state. If the policy for the Low or High state is different from the base case policy, DPL will indicate this with color. Blue means that the policy changed for the Low state of the event, magenta means the policy changed for the High state. The color change occurs halfway between the base case vertical line value and the end of the bar, and does not indicate the exact location of the policy change.

To produce an Initial Decision Alternatives Tornado Diagram, DPL will set the initial decision to its first alternative and then run a Base Case Tornado analysis in the same way. DPL will then repeat this process for each alternative of the initial decision.

The output value of the base case is indicated by a vertical line. The base case outcome value is not the expected value, and the optimal base case policy may not be the optimal expected value policy. In most cases, the ends of a bar will lie on either side of the base case value line. In cases where both ends lie on the same side of the line, the bar will not intersect the vertical base case value line.Be careful reading this diagram when there is extensive conditioning in your model, or if you have events that represent test results or other inference or learning opportunities. Remember that only one event is varied at a time; the others are controlled to their nominal states.

__Value Nodes in Base Case Tornadoes and Initial Decision Alternatives Tornadoes__

You can add bars for value nodes to the diagram by selecting Chart | Series | Data | Modify while viewing the diagram. After you specify a Low and High value for a variable and click OK, DPL will run the analysis and insert the new bar. This bar is, like all others in this diagram, compared to the base case, where all chance events have been set to their nominal states.

__Updating the Diagram__

If you change your model and wish to update the bars in the diagram, select Chart | Series | Data | Refresh. DPL will redo all the comparisons in the diagram using the current model and the previously specified low, nominal and high states for each chance event. If a chance event has changed and has a different number of states, DPL will ask you to confirm the low, nominal and high states for all the chance events in the model.

* Versions:* DPL Professional, DPL Enterprise, DPL Portfolio

*See Also*