USDA Forest Service

Forest Inventory and Analysis National Program


Forest Inventory & Analysis
National Office
U.S. Forest Service
1400 Independence Ave., SW
Washington, D.C. 20250-0003

(703) 605-4177

United States Department of Agriculture Forest Service.

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Data and Tools

Difference Tester Tool
Test for temporal change

[Photograph] Side by side photo of forest condition boundary showing difference over time going from clear cut to new growth trees.  


The Forest Inventory and Analysis (FIA) Program inventories and assesses the Nation's forest resources and publishes the data and findings. Online analytical tools from FIA ( provide current inventory and change estimates that guide management and policy decisions. A broad range of temporal comparisons are of interest to analysts. For example, an increase in the number of standing dead trees can indicate a forest health issue.

Often, these comparisons have covariance because the estimates are derived from observations on the same plots over time (not independent). Specifically, the lack of independence requires that a covariance term be accounted for when computing the standard error of the difference between two attributes. Covariance should be accounted for when conducting hypothesis testing for statistically significant differences (Westfall et al. 2013 at However, in many cases, the only knowledge at hand for each attribute is the estimate and associated standard error. Failure to include covariance in statistical tests can foster incorrect conclusions and poor management decisions.

The Difference Tester accounts for covariance when comparing total (totals version) and per acre (ratio version) estimates of a chosen attribute over time. A calculated z-score and p-value indicate the likelihood that a difference is different from 0 (no difference).

The Difference Tester can test for statistically significant change in forest land area and various tree attributes. Changes in tree carbon, tree biomass, volume of live trees, volume of standing dead trees, tree sawlog volume, number of live trees, and number of standing dead trees can be identified.

A user must have knowledge of the FIA inventory, FIA database, Structured Query Language and statistics to effectively use Difference Tester. The Difference Tester User Guide provides further information and example comparisons.

Click here to run the Difference Tester Tool using totals.

Click here to run the Difference Tester Tool using ratios.

Click here for the Difference Tester User Guide.

USDA Forest Service
Last Modified: October 28, 2019

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