QUESTIONS ABOUT AREA FRAME SAMPLING TECHNOLOGY
Question: What is an area frame?
An area frame (AF) is a special case of cluster sampling. The SUs are areas of land commonly called segments or area parcels, which have identifiable boundaries. The goal is to divide the entire land area of interest into SUs and to select a sample of such segments. The process of area sampling is usually accomplished by selecting the sample in stages, an approach that avoids the necessity of dividing the entire population into segments. An AF is suitable for general purpose sampling and is designed for obtaining information about variables associated with land such as crops, livestock, forests, soil, and ground water. Households inside the segments can also be associated with land.
At the end of AF construction, a small sample of representative land segments or parcels will be selected for data collection. Typically, land within the selected segments account for less than one percent of the country's total land area. In the US, the sample accounts for one half of one percent of the land.
Question: How is an AF constructed?
In order to construct an AF, both maps and satellite images of the land area are used. An AF is constructed as follows:

  • The total land of the country is divided into administrative areas.
  • Satellite imagery is used to subdivide the land in administrative strata into land use strata such as cropland, range, woods, estates, cities and wastelands.
  • Small sampling units (SUs) are constructed using maps. These SUs are numbered and a small sample of land parcels is selected in each stratum to represent the land in that stratum.
The selected SUs are called segments. There are segments selected to represent all strata with land of interest to the data users.
Question: How do you determine sample size?
The sample size is determined by the uses to be made of the data, survey resources, and survey management resources. The sample can be 350 to 3000 segments. These segments will differ in size from one stratum to the next. For example, in range land, segments may contain 400 hectares or more. In cropland, segments may contain 20 to 30 hectares. In cities, the segments may be less than a city block.
Question: How are AF methods implemented?
Implementation of AF methods is straight forward:
  1. Divide the land area of interest into homogeneous farming systems and land use strata;
  2. Subdivide each stratum into (Ni) units of land without overlap or omission;
  3. Select a representative sample of (ni) land units called segments from each stratum;
  4. Collect the desired information from the segments without error; and,
  5. Estimate population totals by multiplying sample totals by the proper expansion factors (Ni/ni).
Question: What are advantages and disadvantages of AF?
Among sampling frames, the AF has the following advantages:
  1. The AF is permanent for 10 years or more.
  2. When employing AF methods, the same segments of land are used for many surveys and thus measure change accurately.
  3. The AF is useful for environmental data as well as agricultural data.
  4. The enumerators can be quality checked with a smaller sample size.
  5. Data can be collected in an integrated fashion.
  6. There are several types of advanced methods of data collection that can be integrated with the AF method that will provide specialized data such as commercial crops and deforestation.
  7. AF data will withstand professional scrutiny.
  8. An AF is complete, has no duplication, and facilitates data collection in the field.
Disadvantages of AF are few, and are primarily related to the expense of setting up the system. However, over a period of 10 years an AF is more cost-effective than other sampling frames.
Question: How can AF technology be used to enhance Geographic Information System (GIS)?
GIS technology allows one to establish logical relationships among layers of information digitized and entered into a computer. For example, roads of a country might be overlaid onto soil maps in order to identify areas where produce can be transported easily.
An essential part of a GIS system is the quality of the data entered. For example, some environmental parameters involve observing the invertebrates in the bottom of a stream. Such minute detail must be collected in a scientific manner before it is useful in a GIS. AAIC personnel are concerned with the quality of data going into the GIS. By using an AF, our data help GIS technology reflect more accurate relationships.
Question: What data can the AF provide?
The AF is designed to collect data from the fields and farmland and from the households that are located inside the segments. For agricultural data, the first survey of each growing season is conducted after planting. Interviewers go to the segments and collect data on the number of hectares planted to each crop. At that time AAIC will recommend collecting data on soil erosion and land use.
Close to harvest, enumerators can go back to these same fields and do crop cutting surveys, which estimate yields and, subsequently, total production. Yield estimates also may be obtained by asking farmers what they harvested (i.e., farmer recall). In addition, we can employ agrometeorological (or agromet) yield models. These models simulate crop growth in the computer. Weather data, rainfall, solar radiation, crop variety, soil fertility, and good historic yield data are required. With several years of data, models are calibrated to conditions in a country and crop yields can be forecasted accurately.
Question: Since animals are mobile, how do you estimate number of heads of livestock?
One can improve data from an AF for specific variables such as livestock by obtaining additional data through list sampling. When data are collected from both the AF and list frames, and combined removing any duplication, we call this method multiple frame sampling (MFS). Most countries improve data with MFS methods.
Question: How are large estate farms surveyed?
MFS, discussed in the previous section, is employed when dealing with estate farms or what we in the US refer to as extreme operators or extreme operations. A list of large farms is prepared and a sample selected in order to represent the list. Data are collected for the smaller farms from the AF and these data are combined in a way that avoids duplication. When MFS is employed, the advantages of list frames (efficient for farms on the list) and AF (complete for all items) are reached.
Question: Can the AF system provide baseline, midterm and end of project data?
An AF can be constructed for a project area that provides baseline, midterm and end of project data. Because permanent plots are established, measures of change are both accurate and cost effective.
Question: Will the AF help reduce the sample size?
The sample size of a survey is independent of the frame used. The sample size has more to do with the level of summarized data produced. When data are required for the smallest administrative levels then sample sizes are driven high because each administrative unit has a sufficient sample. There are better ways to generate small area estimates that do not require increasing the sample past the point where the sample can be managed properly. The AF could also have a large sample and it would therefore encounter some of the same problems that the current system encounters with respect to data quality and management. However, nonsampling errors are easier to control in AF surveys than in list frames.