Translating Stock and Flow Diagrams to Valsight Models

Stock and Flow diagrams can be mapped to Valsight models with simple basic building blocks.

Consider the following, basic example of a stock and flow diagram, adapted from Sterman – Business Dynamics, page 209.

StockAndFlowDiagram

The equivalent Valsight model would look like

StockAndFlow

Assuming a monthly Inflow of 10 pieces and an Outflow of 9 pieces the formulas may look like this:

Node Formula Notes
Inflow EXPAND(10,"Month") You may connect data via the DATA call, make the node simulatable, and use multiple dimensions (products, countries...)
Outflow EXPAND(9,"Month") same as Inflow
Net Change in Stock 'Inflow' - 'Outflow' The delta between Inflow and Outflow. (this node is actually optional, you may as well use the formula directly in the Stock.
Stock RUNNINGSUM('Net Change in Stock') Accumulate the Net Change in Stock. This node must be set to aggregation type "Closing Sum" to show the right end of period value when displayed at e.g. Year level.


Explanation / Details

To understand how this works, let's see which numbers are in each node & month for the first 3 month of 2019.

Month / Node 2019-01 2019-02 2019-03
Inflow 10 10 10
Outflow 9 9 9
Net Change in Stock 1 1 1
Stock 1 2 3

If we now change Inflow and Outflow in a simulation (shown in red), the table may look like this:

Month / Node 2019-01 2019-02 2019-03
Inflow 10 10+4 = 14 10
Outflow 9-1 =8 9 9
Net Change in Stock 2 5 1
Stock 2 7 8

Final Model in Valsight


Setting the aggregation type:

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