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Analysis - Adjusting Sample Weights
Prior to analyzing GRTS Design probability data, it is necessary to ensure that each sampled site is assigned the correct weight. Commonly, weights are adjusted following the completition of the Design Status File to reflect the implementation of the design. Most often the design is not implemented exactly as initially specified, commonly fewer or more sites, thus requiring the weight adjustment.
The initial design weights are assigned during the GRTS design process and individual site weights reflect the amount (stream lenght, estuarine area, etc.) of the target population represented by the site. For example, if the frame contains 1000 km of stream length and the design sample size is 100, then each site has a design weight of 10. If during the evaluation and sampling phase only the first 90 sites are used, then the initial design weights are adjusted:
adjusted weight(i) = sum(design wts)/sum(used wts)xdesign weight(i)
These weights are then used when calculating estimates for the Target Population that is included in the Sample Frame.
- Estimates based on Target Sampled sites apply to the Sampled Population with no additional assumptions.
- Estimates based on Target Sampled sites can apply to the portion of Target Population within the Sample Frame ONLY IF assume that the Access Denied, Target Not-Sampled, etc., sites occurred randomly and independently of site characteristics
- Estimates for Target Population NOT ONLY require assumptions above BUT ALSO that portions of Target Population that are not included in the Sample Frame have same characteristics as the Sampled Population
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