NASS Surveys: The Foundation of Estimates

Types of Surveys

The primary sources of information for NASS reports are those who know the most -- farmers and ranchers, livestock feeders, slaughterhouse managers, grain elevator operators, and other agribusinesses -- who also make extensive use of the estimates. Their cooperation is absolutely vital to a workable and meaningful estimating program; NASS relies on survey respondents to voluntarily supply data for the reports.

The success of this cooperative relationship can be attributed to respondents' recognition of the importance of the survey results and to the confidential treatment NASS accords all data on individual operations.

The information for NASS surveys is gathered in a variety of ways: mail surveys, telephone interviews, face-to-face interviews, and field observations.

The data from a probability-based sample can then be used to make precise inferences about the population. This survey technique has distinct advantages over a census; it takes less time, costs less, and can actually be more accurate because fewer errors are made in reporting and handling the smaller quantities of data. Also, the results of the survey stand alone; they do not depend on relationships to other sets of data, such as the Census of Agriculture.

NASS employs two basic techniques to sample farmers, ranchers, and agribusinesses; area frame sampling and list frame sampling.

The area frame technique can be used to collect information about crops, operator households, animals, grain storage facilities, environmental factors, and so on.

In most States, the segments in the area frame are classified for sampling purposes into four broad categories: land intensively cultivated for crops; land used primarily for livestock production; residential and business areas in cities and towns; and areas devoted to parks, military installations, and other uses.

The area frame sample provides continuous coverage of all agricultural activity in the United States, regardless of changes in farm boundaries and management. This sampling technique guards against omission or duplication in the statistics.

Surveys that rely on the list frame can cost less than those done with the area frame, if the data can be collected largely by mail or over the phone; area frame sampling requires face-to-face interviews.

NASS keeps the list frame as complete as possible, especially for the larger producers, by obtaining records for new or omitted operations from other USDA lists, producer association lists, and other sources.

The probability sample for June contains about 15,400 area segments --roughly 0.7 of 1 percent of the total land area in the 48 contiguous States -- which translates into interviews with roughly 52,000 farm operators. Enumerators also contact between 70,000 and 75,000 farmers and ranchers from the list frame.

After locating each operator, the enumerator explains the purpose and importance of the survey and asks a number of questions. The enumerator carefully records all of the operator's answers. Questionnaires are then sent to the SSO's, where they are checked for completeness and consistency.

Surveys at other times of the year combine a portion of the area frame farms visited at midyear with extensive list samples to obtain data to estimate crop and livestock inventories and production, and acres seeded for fall grain crops.

A major advantage of this program is that it reduces the number of times per year a producer must be contacted for information, thereby conserving resources and minimizing the burden on survey respondents.

Quality controls have been built into the data collection process to ensure the accuracy of survey results. These safeguards include careful selection and training of enumerators, use of detailed instruction manuals, careful field supervision, questionnaire checks, comparison of reported acreage with those measured on the aerial photographs, and follow-up visits to some segments.

With the farmer's permission, enumerators walk a randomly selected number of paces into selected fields and mark off a small sample plot -- no matter what the condition of the crop at that location. This practice minimizes selection bias that could skew the final estimate.

At the designated field location, the enumerator counts the number of plants and measures the distance between rows to determine plant population per acre. Then the enumerator counts immature and mature fruit, such as cotton blooms and bolls: soybean branches, nodes, and pods: wheat heads and spikelets; or ears of corn, and records the crop's stage of development. With the data on plant population per acre and projected yield per plant, statisticians can forecast yield per acre at the State level.

All objective yield surveys except potatoes require enumerators to repeat their visits to the sample plots several times during the growing season. When the crop reaches maturity, they harvest a portion of each plot by hand and send samples to a laboratory for weight and moisture analysis. When the farmer harvests fields containing the plots, enumerators make their final visits to the sample plots to determine harvesting losses and estimate net yields.

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