Reporting and graphing net present value: Understanding Woodstock summary statistics functions

Applies to: Woodstock (all versions)

The Woodstock forest modeling system can produce reports and graphs of outputs over time - even though optional, they are the key to analyzing results. The standard practice is to report and graph output values directly, but you may also use system statistical functions to report and graph maximums, minimums, averages and sums of outputs over time. The purpose of this tech tip is to describe how to produce reports and graphs of net present value using Woodstock statistical functions.

The net present value (NPV) is a commonly used method for valuing a forest estate, and represents the difference between the present value of cash inflows (revenues) and cash outflows (costs) over the projection period. In Woodstock, you declare the net present value as an output, which you can include in reports and graphs. However, Woodstock calculates, and reports and graphs, the NPV in each planning period of the projection, when in fact the NPV is really the sum total of these periodic NPV values.

You can use the system _SUM statistical function to report and graph the total NPV. For example, to declare a graph that plots the running sum of NPV, enter the following in your Graphics section following the *LINES keyword:

_SUM(oNPV) 1

This instructs Woodstock to calculate a running sum of the output oNPV and plot it in the first graphics window. The NPV of the forest is the value plotted in the last planning period.

Similarly, you can use statistical functions in reports. To report the NPV, enter the following in your Reports section:

*TARGET Value.txt
_SUM(oNPV) 1.._LENGTH

This instructs Woodstock to calculate a running sum of the output oNPV and write it to the report file Value.txt. Again, the NPV of the forest is the value reported in the last planning period.

In addition to sums, Woodstock also includes statistical functions that you can use to report and graph minimums, maximums, and running averages of outputs. You can read about Woodstock statistical functions in Chapters 11 and 12, Reports Section and Graphics section, respectively, of the Woodstock Users Guide.