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NRI Reliability

NRI Data Reliability

NRI is a statistically-based survey that has been designed and implemented using scientific principles to assess conditions and trends in the state of our natural resources.  The National Resources Inventory is conducted in cooperation with the Iowa State University Center for Survey Statistics & Methodology (CSSM).

Margin of Error computations are automatically generated with most NRI data analyses using the U-CARP statistical package developed as part of a cooperative agreement with Iowa State University.   A Margin of Error defines the confidence interval associated with each estimate within the report.  All Margin of Error estimates are for the 95% confidence level.

A confidence interval/margin of error has a convenient interpretation.  Specifically, for a 95% confidence interval, the probability that the interval covers the true mean value is equal to 0.95.  This means that if we draw an infinite number of samples for a specified population (e.g. cropland acres, erosion rates, etc.), and if for each sample we compute the confidence interval, then 95% of those intervals will contain the true mean.  The measure "95%" represents the degree of our confidence that the interval contains the true population mean.

To make this concept more concrete, consider a watershed.  We want an estimate of the number of acres of cropland in this watershed.  The number of acres of cropland constitutes our population.  From the NRI data, suppose that we estimate the number of cropland acres to be 50,000 and the confidence interval to be (47,000-53,000).  This means that the probability that the actual number of acres of cropland in the watershed de facto is between 47,000 and 53,000 is 0.95.

The sample design for the 1997 NRI is designed to provide acceptable margins of error at the 95% confidence level for geographic areas based on USDA Major Land Resource Area definitions.  Analyses based on smaller geographic areas may produce results with meaningless margins of error.

Link to the National NRI website on Statistical Estimation that provides appropriate scientific citations.