Area Frame

Plot and Grid Area Frames

There are two types of area frames: grid frames and plot frames. The distinction between a grid and plot frame is based on the analytical goal of the survey rather than the structure of the frame. Plot frames contain ultimate sampling units that are observed in their entirety, whereas grid frames contain land units that will be further divided and sampled at further stages.

Plot frames are often used in agricultural and environmental surveys in which measurements are taken on a piece of land. For example, consider a survey designed to estimate pollutants in a stream. After obtaining a map of the stream, one could partition the stream into 3-foot-by-3-foot square plots. If a sample of plots is selected and the pollutants in each sample plot are measured, then the map of 3-foot-by-3-foot square plots is a plot frame, because the entire plot is enumerated.

Sometimes is it desirable to select a sample of units within geographic areas. In grid frames, geographic clusters of sample units compose the frame. The geographic clusters are first sampled. Then a sample is selected from units within the sampled clusters.

Use of Area Frame in Multi-Stage Sampling

Grid area frames play a central role in multi-stage sampling. At every stage of selection except the final stage, a different area frame is used. For example, consider a survey designed to estimate the median income of all households in a city. In the United States, one possible area frame for the first stage of sample is a list of all census tracts. After selecting a set of tracts, one could construct a second area frame of all census blocks within the selected tracts. Blocks that are not in selected counties are not considered a part of the sampling frame because they do not have a chance of selection.

Before selecting the final stage of households in sample blocks, a list of households within the blocks needs to be built. Field staff often perform this role by listing all households within the selected blocks; although the list of addresses could be obtained from an administrative list. In the final stage of sampling, the list of housing units is an example of a list frame rather than an area frame. However, sometimes geographically clustered lists built from a field enumeration are referred to as an area frame.

Reasons to Use Area Frames

When a satisfactory list frame is not available, an area frame may be the best alternative. For example, consider a survey of homeless adults in a large city. In the absence of a list of homeless people in the city, one could construct an area frame of city blocks that would cover the entire population. In such a case one might also want to use a second frame of people staying in a homeless shelter to supplement the area frame.

...