Sample Size and Database Requirements for Resampling and



Sample Size and Database Requirements for

Resampling and Reporting

BLM's Attainment of the National Riparian PFC Goal

for:

USDI Bureau of Land Management

Washington, DC

by

Paul R. Adamus

Dynamac International, Inc.

Rockville, MD

September 1998

Background

As part of the national Riparian-Wetland Initiative (USDI Bureau of Land Management 1996), the BLM intends to implement a goal of improving, by the year 2005, 90 percent of the "Functioning -- At Risk" riparian miles in the 48 conterminous states. Consequently, BLM needs to know if at least 90 percent of these miles have shifted from the "Functioning -- At Risk" (F/AR) category to the "Proper Functioning Condition" (PFC) category. These categories and the protocol used to determine them site-specifically are explained in Pritchard (1995).

It is projected that of all the riparian miles assessed in the 48 contiguous states by the end of FY99, there will be 17,300 miles that fall in the F/AR category (E. Luse, pers. comm.). BLM's need to know if at least 90% have shifted to the PFC category should not be interpreted to mean that 90% of the miles will be in the PFC category then, but rather that 90% (15,570) of the 17,300 reaches that in 1999 were F/AR will shift by the year 2005 to the PFC category. For example, the number of miles in the PFC category might shift from 14,680 (in 1999, the recognized base year) to 30,250 (in 2005), but this still would represent only 76% of all 39,905 miles assessed by the end of FY99.

The primary purpose of this report is to present a pilot assessment of the number of riparian miles that would need to be resampled to achieve various hypothetical goals for precision in estimating PFC trends.

Results of Pilot Assessment

The number of miles that would need to be resampled depends partly on how certain one needs to be that 90% (not, say, 40% or 60%) of the miles have indeed changed -- in other words, what confidence interval (margin of error) do we desire?

Also, BLM may be interested in focusing on a subset of the 17,300 miles that have been subject to new grazing management strategies since 1999 -- hypothetically, 5000 miles that were F/AR as of 1999.

Finally, BLM may be interested in determining whether the 90% goal has been attained in particular states or river basins, so another subset (say, all 1000 miles comprising F/AR in a particular state) might be requested.

Table 1 shows results of our calculations, assuming each of 2 confidence intervals (.20 and .10 -- i.e., 90 and 95% certainty of estimate) and 3 sample sizes (17,300, 5000, and 1000 miles). Appendix A contains documentation of the equations and references used to produce Table 1.

Table 1. Recommended sample sizes for PFC reassessments of Functioning - At Risk (F/AR) streams on BLM lands, 90% goal

|Desired |Number of |Required number of F/AR miles that should be resampled |

|Confidence Interval |F/AR miles anticipated to exist in | |

|around 90% goal |1999 base period | |

|(i.e., allowable margin of error) | | |

|+/- 5% |17,300 |140 |

| |(1999 anticipated) |(1% of all F/AR) |

|+/- 10% |17,300 |35 |

| |(1999 anticipated) |( ................
................

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