A statistical textbook definition of a frame is; the materials or devices which delimit, identify, and allow access to the elements of the target population. The frame also includes auxiliary information that is used for special sampling or estimation techniques.

In NASS, our target population is all farm operators. We maintain a rather comprehensive and nearly complete list of the farm operators, but we do not know about all of them at any given point in time. There are changes in farm operations (additions, deletions, and change) on a regular basis. We do know that all farms occupy space or land area. Therefore, we maintain an area frame or a frame based on land area as a device to allow complete access to our target population. We can contact the farm operators by selecting blocks of land, visiting these blocks, and finding any farmers that operate on that land. To see an example of a sampled block of land called a sample segment, click here .

NASS can use the information gathered from the area sample several ways. First, the information can stand alone to estimate agricultural activity. It is particularly effective for estimating the number of acres planted to crops. Second, we can combine list based and area based samples in an optimum fashion using multiple frame statistical estimators to measure many items of interest. This multiple frame approach takes advantage of the strengths of both methods of sampling farm operations. Finally, we use the area sample to provide coverage measures for the Census of Agriculture. 

NASS has used area frames for agricultural surveys since the early 1960's. We maintain an area frame for the 48 conterminous states. The frames are constructed by defining blocks of land whose boundaries are physical features on the ground (roads, railroads, rivers, etc.). These blocks of land cover the entire state, do not overlap, and are placed in strata based on the percent of land in the block that is cultivated. These strata allow us to sample the land very efficiently, that is, we are more likely to find farms in areas with greater that 50% cultivation than in cities or rangeland areas. To see examples of area frame stratification for selected states click here . We will sample more land from the areas with a higher percentage of cultivation than a lower percentage. Each year NASS replaces some of the area frames because the land use changes over time.