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Development of the Sampling Frame
The list or map that identifies every unit within the target population is the sampling frame. Such a map or list is needed so that every individual member of the population can be identified unambiguously. The individual members of the target population whose characteristics are to be measured are the sampling units. The sampling frame would typically be established by combining one or more sets of frame materials through a geographic information system (GIS). Frame materials usually encompass several information sources, for examples see Frame Materials.
- Sample Frames almost always are not exact representations of the target population
- Sample Frame may not include some Target Population elements: Undercoverage
- Sample Frame may contain non-target elements, e.g., mis-identified sample units: Overcoverage
Also see: Relationships among Target Population, Sample Frame, Samples, Sampled Population, Estimates, and Assumptions
Streams - Example
GIS coverage that includes all streams in the target population
Potential sources: River Reach File Version 3 (RF3), National Hydrography Dataset (NHD)
- RF3 & NHD based on 1:100,000 scale USGS maps
- Excludes some channels that appear on 1:24,000 USGS maps and not on 1:100,000 maps
- Includes some channels/features that are not streams
Other GIS coverages may provide additional required attributes
Lakes - Example
- GIS coverage of lakes and reservoirs in State, Region, US
- RF3; NHD; State lists/coverages
- Lakes: two alternatives for elements:
- Each lake is element: lake is viewed as a point
- All points in all lakes are elements: area view
Estuaries - Example
- GIS coverage of coastal waters in study area
- Estuary open water
- Tidal streams/Rivers
- Near-shore waters
- Elements are all point locations within target population