Frames for agricultural censuses and sample surveys
An agricultural census is an agricultural survey in which the estimates are based on the complete enumeration (100 percent) of all holdings. A questionnaire would be completed for each holding.
Agricultural sample surveys are those agricultural surveys that are not censuses. Thus, the estimates of the specified variables are obtained by inferences based on a sample of all reporting units (holdings or land areas). Questionnaires would be completed for a sample of reporting units.
A non-probability or subjective sample survey is an agricultural sample survey for which the inference procedure to obtain the estimates of the desired variables is not based on probability sampling and estimation methods. A subjective agricultural sample survey is, therefore, a survey in which the statistical precision of the estimates cannot be established.
Conversely, an agricultural sample survey for which the inference procedure to obtain the estimates of the desired variables is based on probability sampling and estimation methods is called a probability sample survey. For a probability sample survey, the statistical precision of the estimates has a clear mathematical meaning and can be established.
The overall survey design of an agricultural probability sample survey refers to the definitions and the established methods and procedures concerning all phases needed for conducting the survey: the sample design, the selection and training of personnel in charge of the specified data-collection procedures, the organization of the logistics involved in the distribution and receipt of survey forms and questionnaires, the data-processing procedures and the analysis of survey data needed for the release of the final results.
The sampling design of an agricultural probability sample survey refers to the definitions and the established methods of sampling selection and estimation. There is a variety of agricultural sampling designs involving different types of sampling unit, probability functions, sampling fractions, stratification and clustering procedures and estimators. There are also designs that involve several sampling stages, in which case for each sampling stage the sampling units, probability functions and sampling methods have to be established and combined to obtain the last-stage survey estimates.
The agricultural survey can have a variety of sampling designs and can be carried out in a number of ways with different enumeration procedures (by personal field interview, by expert observation, self-administered questionnaires, by mail or, perhaps, by telephone), which may or may not include the identification and objective measurement of agricultural areas or other agricultural commodities in the selected enumeration units.
As has already been mentioned, the national periodic agricultural surveys considered can be agricultural censuses or agricultural sample surveys.
There are two basic types of agricultural probability sample survey: area sample survey and list sample survey.
An area sample survey is a probability sample survey in which the final stage sampling units are land areas known as segments, and the selection probabilities are proportional to their area measures. For an agricultural area sample survey the entire survey area is completely subdivided into non-overlapping land areas, or segments, that are the sampling units.
For probability area sample surveys, the most common types of segment (last-stage sampling units) used are: segments with recognizable physical boundaries, square segments or segments that coincide with agricultural holdings.
A tract is a portion(s) or subdivision of a segment that is included in a holding. It is either (a) an entire holding; (b) part(s) of a holding; or (c) an area of land not part of any agricultural holding.
A tract is therefore determined by the definition of a holding and by the boundaries of a segment.
A holding is composed of one or more tracts. All land under the operating arrangement - non-agricultural land, farmstead, barns, corrals, pasture, ponds, etc. - is included in the tract. A questionnaire should be completed for each tract, except for those tracts that consist of wasteland, water and non-agricultural land that will usually be listed only on a control sheet and not subjected to a questionnaire.
The reporting units of an area sample survey are usually the holdings or the tracts.
Three main methods have been used in area sample surveys to associate reporting units (holdings or tracts) to the sampling units (segments) for estimating survey variables: the closed segment, the open segment and the weighted segment methods. The weighted segment method and also the open segment method correspond to surveys and estimates for which the reporting units are the holdings. The tract is the reporting unit for area sample surveys with a closed segment procedure.
A list sample survey is an agricultural probability sample survey that is not an area sample survey. The usual reporting units of agricultural list sample surveys are the holdings and the commonly used final-stage sampling units are the holdings' addresses or the holders' (farmers') addresses.
Finally, multiple-frame agricultural surveys are the agricultural surveys that combine the two basic types of probability sample design (area and list samples).
The frame for an agricultural census is a set of physical materials (lists, directories, records, maps, statistics) that covers the universe of reporting units and allows the provision of suitable enumeration units.
For a probability survey, the term "frame" has two different meanings. In probability sample design, the frame for each sampling stage is simply the list of sampling units from which the sample is selected together with their probabilities of selection.
On the other hand, for applied survey sampling, the frame for each sampling stage of an agricultural sample survey is a set of physical materials (lists, directories, records, maps, statistics) that covers the universe of reporting units and allows the provision of suitable sampling units.
The agricultural census frame and the agricultural sample survey frames are discussed separately in the following sections in relation to the improvement of population data that can be obtained from agricultural data collection programmes.
The reporting units and the enumeration units of an agricultural census are the agricultural holdings. In order to organize the agricultural census data collection, it is indispensable to estimate in advance the approximate location of the agricultural holdings or the holders' housing units and to assign to census enumerators well-defined areas of work, known as enumeration areas (EAs).
The enumeration areas are non-overlapping geographic areas (or groups of enumeration units) with the following characteristics:
· They cover all the enumeration units (agricultural holdings or holders' households).
· If the EAs are geographic areas, their boundaries should preferably not cross urban, rural or political subdivisions of the country.
· The area (or number of enumeration units) of the EAs should correspond to approximately equivalent required workloads. They can be estimated by: the approximate number of holdings or holders' households; the distances and difficulties of access to the holdings or holders' housing units; and the average time needed for each interview and the established length of the census enumeration period.
An agricultural census frame is defined as an ordered list of enumeration areas with the estimated number or addresses of holdings, or holders' households or holders' housing units in each EA.
National population and housing census data and cartographic materials often provide an important source to construct the frames for the agricultural census. This is one of the reasons why it is of fundamental importance to coordinate adequately the population and the agricultural censuses which, in most countries, are the largest, most expensive and most complex statistical data collection programmes.
It is important to coordinate the designs of the population and the agricultural census cartography or lists of EAs, or to elaborate them jointly, because this will save a considerable amount of time and resources. This is particularly significant for countries that can devote only very limited resources to national statistical data collection programmes. and to countries with a high proportion of population in the agricultural/rural sector.
The enumeration unit of the population and housing census is the household and not the agricultural holding, which is the enumeration unit of the agricultural census.
Population census questionnaires, in most cases, do not allow a direct link to be established between holdings and holders. However, the population census questionnaires identify the population whose main activity is agriculture within the context of a fairly short reference period.
The universe of study of the population and housing census, i.e. the total number of households/housing units in the country, is larger than that of the agricultural census since the latter covers, in particular, only a small proportion of urban households.
In urban areas in which the proportion of holders' households is generally low, in order to construct the agricultural census frame a field screening is often organized to permit the identification of the holders' households and of the agricultural census EAs by grouping contiguous population census EAs.
In rural areas, if the number of holders' housing units can be estimated from the total number of occupied housing units by EA, then suitable groups of contiguous population census EAs can be used to define the agricultural census EAs.
Consideration should be given to the following aspects of the population census questionnaires with relation to the construction of the frame for the agricultural census:
· The inclusion of questions aimed at identifying holders' households, such as obtaining the main and secondary occupation of the population during the agricultural year. And questions which differentiate between work performed on the respondents' holding and work performed on other holdings.
· The inclusion of questions designed to provide information about some basic characteristics of holdings.
The definition of work performed by men and women on the holding, as has already been mentioned, should be carefully applied since women's activities on the holding may be more difficult to detect for cultural reasons.
The population census questions that distinguish between work performed on the respondents' holding and work performed on other holdings and those questions that allow the identification of holders' households are generally essential for constructing the agricultural census frame.
The construction of an agricultural census frame must also cover the holdings related to agriculture that are not directly associated with holders' households, such as large plantations or cooperatives. Although the population census may provide very useful data, it should not constitute the sole source of information for the construction of an agricultural census frame.
The frame for each stage of selection of an agricultural survey is defined as an ordered list of sampling units, together with their assigned measures of size. The enumeration units and the ultimate sampling units are the holdings, the holders' households, the households or the housing units.
The frames for the last stage of selection of an agricultural list sample survey generally involve the screening of agricultural holders from a list of households in selected areas. Therefore, the same considerations that are taken into account in the construction of a frame of an agricultural census are valid, in particular those intended to minimize gender biases.
Population and housing census data, enumeration areas and maps often constitute an important component in the construction of agricultural list sample survey frames. In addition, in most cases the population census information is the best tool for detecting inconsistencies and verifying the validity of agricultural sample survey and census frames.
In view of the above, the proper identification of holders' households using the population and housing census is often used for the construction of an agricultural list sample survey frame. For that purpose, the application of an adequate definition of work in agriculture performed by men and women should be considered.
As in the case of an agricultural census, the construction of an agricultural list sample survey frame must also cover the holdings related to agriculture that are not directly associated with holders' households.
A sampling frame for each sampling selection stage of an agricultural area sample survey is an ordered list of land areas, called "frame units", with their assigned measures of size which are the number of segments in each frame unit, with the following characteristics: (a) the frame units form a complete subdivision of the total land area with no overlap; (b) the frame units provide a clear-cut means of identifying each sample unit; and (c) the measure of size assigned to each frame unit facilitates the probability sampling of sampling units.
Hence, except for the case in which the segments are the holdings, the area sample survey frame construction is usually unrelated to the holdings.
For area sample surveys, the annual or seasonal data collection in the sample segments is done by enumerators who apply a questionnaire through direct personal interviews with the selected holders or respondents. The area sample surveys generally involve, in addition, the objective measurement of agricultural areas. The enumerator, during data collection, identifies and delineates on aerial photographs (or scale maps) the parcels of land of each tract in the segment, which are later measured in the office.
A tract within a segment is often divided into a number of fields in which the land use is different. On the photo enlargement or scale drawing of the segment, tracts are given letters and fields are given numbers consecutively within each tract.
A field is a block of land, recognized by the farmer, that is usually treated as a unit. It may be all planted to the same crop or the same mix of crops in any one season or it may be subdivided into different crops, but the boundaries are not changed. The crops may be changed for the next planting but the outermost boundaries or "edges" are maintained. This is a useful concept where the farmers think and work along these lines. Fields are usually recognizable on photography and even on imagery where fields are large.