Page 674 - ExtendSim User Guide
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648 Analysis
Sensitivity analysis
Ensure that the Run > Use Sensitivity Analysis command is checked (it is checked by default).
Run the simulation. ExtendSim runs the simulation four times, from run 0 to run 3. In the MultiSim Plotter, click the AutoScale Y tool
to rescale the vertical axis.
You can see the variation between the runs in the four plotted lines. As expected, increasing the stream’s flow increased the amount of water in the reservoir. In more complex models, the effect of making a change would not be so obvious.
Specifying the sensitivity method
The Sensitivity Setup dialog lets you specify that a sensitized parameter will change from a list in a file, incrementally, randomly, or from based on the value of a database field. The choices are:
Multisim Plotter results
Read from col- umn x of file
Starting at... change by...
Random distri- bution
Database field
Assigns the values from a text file. This is the option you will most likely use when performing ad-hoc experiments. If the file has more than one column sep- arated by tab characters, specify the desired column. Starting with the first row of the specified column for the first run, this option uses the value of each suc- cessive row in that column for subsequent runs.
Specifies the starting value and the amount of change. By default, the starting value is the same as the parameter’s value in the dialog. Increase the variable with a positive number or decrease it with a negative number.
Uses a random distribution to set the parameter. This is an easy way to make a single value in a model change randomly over many simulation runs while keeping the value constant within a single run. Choose from one of the five types of distribution and enter the distribution parameters in the options to the right of the distribution. The seed is the number to use for the random number generator. As in the Simulation Setup dialog, BLANK or 0 for the seed is ran- dom.
Assigns values from the fields in an ExtendSim database. Starting with the first record of the specified field for the first run, this option uses the value of each successive record for subsequent runs.
☞ If you are a developer, you can use the CurrentSense variable to control the order of use for the values set for specific runs from within a block. For example, you can cause the first simula- tion run to use row 4 of a file, rather than the default use of row 1.
Turning sensitivity on and off
You can control sensitivity globally (for the model as a whole) or locally (at the dialog param- eter level for a specific block):
• Use the Run > Use Sensitivity Analysis command to turn sensitivity analysis on and off for the model as a whole.
How To

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