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Determining Sample Sizes

Explicit description of objectives and criteria are required establishing sample sizes.  In addition, information on known or suspected implementation issues (frame errors, access, etc.) can be used to increase the likelihood of successful implementation.  Also see, frequently asked questions on determining sample size and why 50 samples.

Determination of sample size requires knowledge and consideration of:

Establishing sample size(s) requires detailed information on:

A reality faced in most studies is:

Sample size calculations for the estimation of a proportion, e.g., proportion of stream length that meets a designated use depend on:

Oregon State Example - Precision Goals and Results

Base Samples

Sample Size 50

Sample Size 25

Sample Size 250 (potential result with 5 year rotating panels)

Over Samples

When known or suspected circumstances are likely to prevent sampling at some base sample sites, e.g., frame errors, denied access, hazardous site conditions, etc. prior additional sample sites can be identified.  These Over-Sample sites are sampled whenever a Base-Sample site cannot be sampled.  This strategy greatly increases the likelihood of attaining the desired sample sizes and fulfillment of the objectives.  Often an additional 10-30% sites are selected as Over-Sample sites.  See Implementation Issues for examples from existing programs.  Go to Top

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