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<<<< | >>>> | Build a VORSIM model – a 5 step summary
If you want to build a simulation model in a spreadsheet, VORSIM can make the job much easier and quicker in terms of the mechanics. Model construction consists of the creation of three Excel workbooks (definition, data, and equation-parameter), the entering of information into these workbooks, and the generation of a stand-alone model workbook. The first step in building your own model is the design of its organization and equation structure. What will be the time span? What are the major model divisions (Sheets)? What are the subdivisions (Categories)? What are the types of variables? How will the simultaneous equation model be closed (or how will equations be linked across sheets)? Sketch out your core model structure, choose VORSIM compatible codes and proceed as follows: 1. DEFINE A NEW MODEL FROM THE VORSIM MODEL BUILDER CONTROL SCREEN: Begin by clicking the [Create New Model] button on the VORSIM Control Screen to bring up the menu for entry of the model name, storage drive, time span, and structure (codes for Sheets, Categories, and Variables). When done, the new model {Definition} sheet will be created as the first sheet in the <Definition> workbook. Fill in descriptions, format codes, and lags for variables. Then click the [Complete Model Definition] button at the bottom of the {Definition} sheet, creating the {Variables} sheet. An alternative way of proceeding is to make a copy of any existing model using the “Duplicate Model” utility. You can then edit this new model definition workbook manually to enter the codes that you want.
2. 2. CREATE MASTER VARIABLE LIST, DATA AND EQUATION WORKBOOKS, AND ENTER DATA INTO THE DATA WORKBOOK: Remove any 1’s where you don’t need variables on the {Variables} sheet. Next, click the [1. Create Variables] button to enter variable names into the master variable list. Then click the [2. Create Data Workbook] button and enter your data into the new <Data> workbook. Finally, on the {Variables} sheet of the new <Definition> workbook, click the [3. Create Equation Workbook] button to create a new <Equation> workbook. Load the new <Equation> workbook with the [Load Equation-Parameter Workbook for Model] button on the VORSIM Control Screen 3. WRITE MODEL EQUATIONS IN THE EQUATION WORKBOOK: From the {Basedata} sheet of the <Equation> workbook, click the [Update Basedata sheet for Model] button to bring in base data from the data workbook. Then go to the first sheet where you want new equations and click the [Add Equation Parameter Matrix] button to bring up the equation creation menu. Fill in the information and click the [3. FINAL STEP – Create Equation Parameter Matrix] button on the menu to create the equation matrix. Repeat this process (with the [Add Equation Parameter Matrix] button) to create all of your equations on the sheet. On other equation sheets you can repeat this process for new equations. If you want to copy some or all of the new equations you have created to other sheets, there are several options. If you bank the new equations with options on the equation bank menu (brought up by clicking [Equation Bank] button), you can restore some or all equations from the bank or from other sheets. This means that if you have added new equations to one sheet, the set can be easily duplicated onto other sheets. The [Edit Equation] button brings up an equation-editing menu if needed. You can also edit equation terms (column heads) manually. Once equation matrices have been added, click the [Write * Equations] button on the sheet to write equations. Alternatively, you can go to the {Variables} sheet and click the [Write -- ALL Equations] button to write all of your equations. 4. CREATE MODEL WORKBOOKS FROM THE EQUATION WORKBOOK: From the {Variables} sheet of the <Equation> workbook, click the [1. Create Model Workbooks] button to create the <Model> workbooks and then click the [2. Put ALL Equations in Model] button to put all of the equations into the model. Then close all open workbooks and from the VORSIM Control Screen, click the [Model] button to load the new model. Go to the {Model} sheet to configure the model for solution. 5. CONFIGURE THE MODEL FOR SOLUTION: Click the [1. Numbers (white cells) to change for model solution (drive (yellow) target cells to zero] button and select cells to be changed by the Solver to find a solution. Then click [2. Formulas (yellow) to drive to zero for solution] button and select target formulas. Note that the number of target cells has to equal the number of change cells. Depending upon your model, you also may want to invoke some of the configuration options such as placing bounds around the change variables. One useful option for a large model is the [3. Option: Select (yellow) aggregate target cell (formula = 0 when model solved)] button. This provides an overall target for the Solve to reach for. Finally click [4. Install Solver] button to install the Solver and save the model. Buttons 1-3 and others as well as the {Solver parameters} sheet will be removed and the model is ready for testing and simulation. Click the [Solve Base] button to solve the model for the base time period (you may get a non-feasible solution notice – this may mean a problem for you model or it may mean your convergence criteria are too tight). |
SOLVING THE MODEL AND SAVING SOLUTIONS: To solve the model: Click the [Solve **** model] button to solve the model for all time periods and use the [Save Solution] button to save the solution with a name of your choosing. You can view or change exogenous variables (one at a time) for solutions with [Change Exogenous Variable | View Solution Variable] button. An alternative option for changing exogenous variable values is to use button [a) Load time series exogenous data workbook (****exot.xls) for exogenous data entry] to enter all exogenous data and then use button [b) Return exogenous data entered manually in the ****exot.xls workbook to the ****exog.xls exogenous data workbook for model simulation]. For static models, you simply change exogenous variable values in the model sheet itself. VORSIM control button names plus occasional associated messages guide you through most operations. Where a sequential set of operations is needed, buttons are numbered in the order they should be clicked. Note that in VORSIM 15, the file suffixes would be *.xlsx. |