Workshop on Area Sampling Frame Key features of area sampling frame Hassan Serghini Budapest 15-18 October 2012 This presentation is preparation by using references mentioned at the end of this presentation NOM Prénom Fonction

The Concept of Area Sampling Frame 08/12/2018 CONTENTS: Introduction The Concept of Area Sampling Frame Conditions for the application of the ASF Requirement for an area sampling frame Steps for constructing an ASF Key features of the area sampling: Versatility Key features of the area sampling: Coverage Key features of the area sampling: Updating Weaknesses Strengths Conclusions

Introduction For probabilistic samples we need a Frame where we can identify all statistical units of the population under investigation; A frame should not contain any unit that does not belong to the population to be studied; A frame should be without any duplication or omission; For the frame to respond to these characteristics, should be updated before the sampling.

The Concept of Area Sampling Frame The area sampling frame (ASF)concept is very simple: Total area to be surveyed is divided into N small blocks, No overlap or omission; Selection of a random sample of n blocks; The simplicity of the concept is in contrast with the complexity of its successful application;

Conditions for the application of the ASF 3 important conditions involved in the application: The reporting units must be defined to serve the purpose of the survey, Need of practical means to associate reporting units with the area sampling units, area sampling should be better than alternative feasible sample survey methods.

Requirement for an area sampling frame Availability of up-to-date cartographic material (maps, digital maps, satellite images, areal photos, GIS software) covering all the frame. Detail of the material must be sufficient to allow: Stratification according to proportion of land cultivated, predominance of certain crops or other uses of land, etc., subsequent subdivision of these land-use strata into areas called primary sampling units (PSUs) with recognizable permanent physical boundaries.

Steps for constructing an ASF Stratification based on land use & likelihood of finding agriculture Divide strata into strata blocks Select a sample of blocks Divide selected blocks into segments Select a sample of segments; Keep segments in sample for 5 years, rotate 20% of sample each year

Key features of the area sampling: Versatility Possible uses of area sampling are unlimited. Reporting units uniquely associated with segments: households, persons, farms, plants, animals, suppliers of agricultural input, tracts of land, processors of agricultural products… Adaptability to particular uses, and versatility, are strong attributes of area sampling. Sometimes area sampling is the only mean available for selecting a probability sample.

Key features of the area sampling: Coverage Conceptually, an area sampling frame is always current and complete with regard to any definition of a reporting unit. Total population: N segment, sample n segments ∑X, is the total of characteristic X for all reporting units associated with the sample. N/n ∑X is an estimator No need to know the number of reporting units in the population in order to apply area sampling.

Key features of the area sampling: Coverage One estimator of the number of reporting units in the population is N/n(r) , where r is the number of reporting units in the sample of n segments Area sampling frame is conceptually complete. However in practice, coverage error is a major problem: the identification and the association of reporting units of each sample segment might be Incomplete, or not done correctly, n/N will not be the actual sampling fraction. N/n ∑X will not be an unbiased estimate of the population total.

Key features of the area sampling: Updating An area frame does not become out-of-date in terms of coverage of a population. Changes in land use, or number and location of reporting units, have a bearing on the sampling variance but do not introduce bias. Some boundaries of sampling units will lose identity as time passes: potential for bias as a result of greater ambiguity about boundary locations.

Key features of the area sampling: Updating There are two possible reasons for updating an area frame: Maintain or achieve improvements in sampling efficiency, or Introduce updated or new maps to achieve better boundaries of sampling units.

Weaknesses Coverage Updating Non-sampling errors Sampling variability Sampling unit definition Cost List Does not cover entire population out-of-date quickly, frequent updating increased non-sampling errors ASF difficult to target rare commodities sensitive to outliers Outliers & rare commodities & inefficient for farm variables Physical boundaries Expensive and cartographic material

Strengths Coverage Updating Non-sampling errors Sampling variability Sampling unit definition Cost List Target rare commodities reduce Uses mail and telephones and cost efficient ASF Complete (holdings and tracts) Longevity, versality Reduced Good estimates for large variables

Conclusions A complete up-to-date list of farms, is highly desirable for sampling purpose and has strong advantages with regards to sampling efficiency and cost. The cost of establishing a list frame, in many cases is very high (agricultural census) The coverage of list frames rapidly becomes out of date. Area sampling is often needed because of deficiencies or the absence of list frames.

Conclusions If list frame is nearly adequate or list frame covers part of the population: multiple frame (list of large farms); If adequate list frame not available: area sampling is the only possibility for selecting a probability sample. In many developing countries, the areas reported by farmers are considered non reliable: area sample can reduce reporting errors An area frame is always conceptually complete. An area frame is durable and can be used to gather all types of information associated with a farm or the land

References used Earl E. Houseman, Area Frame Sampling in Agriculture, Statistical Reporting Service • U.S. Department of Agriculture, SRS No. 20; FAO, Multiple Frame Agricultural Surveys, Volume 1, Current Surveys Based on Area Sampling Methods, FAO Statistical Development Series, FAO LIBRARY AN: 375713, Rome 1996; FAO, Multiple Frame Agricultural Surveys, Volume 2, Agricultural Survey Programmes Based on Area Frame or Dual Frame (Area and List) Sample Designs, FAO Statistical Development Series, FAO LIBRARY AN: 401697, Rome 1998;

 شكرا Thank you Merci