Transcript Mapping and preparation of frames and Sample design

Technical review meeting on World Programme for
the Census of Agriculture 2020
Volume 2 – Operational guidelines on implementing census of
agriculture
Rome, Italy
30-31 January 2017
Chapters 12 and13:
Mapping & preparation of frames and
Sample design
Item 4
Oleg Cara
Consultant, Agricultural Censuses and Surveys Team
ESS FAO
Naman Keita
Consultant, Agricultural Censuses and Surveys Team
ESS FAO
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CONTENTS
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Mapping
◦ Purpose of maps used for Agricultural Census (AC)
◦ Cartographic work and types of maps
Preparation of frames for AC
◦ Types of frames and sources of information
◦ Some guidelines on building frames
◦ Frame requirements for various census modalities
Use of sampling for census enumeration
Main types and choice of sample design
Advantages & disadvantages of complete
enumeration and sample enumeration censuses
Discussion points
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Purpose of maps
Census planning and frame preparation
o
Provide the cartographic base for planning (including budget estimation of)
the census.
o
For a sample enumeration can be used to improve sample design by
providing information for stratification and other auxiliary information.
Organization, conduct and supervision of field data
collection
o
Maps (or sketches) are used to organize the field work.
o
GIS provides a computer-based design of EAs and significant automation of
map production tasks.
o
The use of maps should be an essential part of the enumerators' training.
Presentation and analysis of census results
o
Maps can be used to relate statistical data with the corresponding
geographic area, facilitate the understanding of statistics and assure a more
extended and appropriate use of data.
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Cartographic work and types of maps
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The cartographic work should be conducted during the early stages of
preparatory census activities.
•
It should start with the inventory of available geospatial information
and maps and the evaluation of their suitability for AC purposes.
B
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A large proportion of the cartographic preparations for an AC consists
of delineating and identifying the enumerators' areas (EAs) of work.
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In many countries EA maps are prepared as part of the cartographic
work of the Population and Housing Census (PHC). In some countries
the cartographic work is conducted for both PHC and AC (e.g. Brazil).
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Adapting, revising and updating available maps to the census
requirements.
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Types of maps: topographic sheets, other government maps, satellite
images, aerial photography, communication maps, land-use maps,
economic maps, city and tourist maps, EA maps from PHC.
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Types of frames
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The List Frame is a list of agricultural holdings and/or
households. However, when the list of agricultural holdings
and/or households is missing, the list of EAs could serve as
a starting point for the development of a census frame.
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The Area Frame is a set of land elements, which may be
either points or segments of land.
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The Multiple Frame, in which part of the population is
covered by a list frame (e.g. special holdings) and the
remainder (e.g. other holdings) by an area frame.
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Sources of information for the
construction of an AC frame
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The statistical data and maps of the EAs of the most recent PHC
and a list of EAs along with their number of households/housing
units.
The statistical data and maps of the EAs of the latest AC
including a list of EAs with their number of holdings.
Statistical farm registers, cadastral and other cartographic
materials, other statistical and administrative data sources, which
include listings of holdings and/or holders' addresses or their
approximate location.
It might be necessary, due to lack of reliable information, to
prepare the census frame by conducting a listing operation (prior
to starting the enumeration of the holdings).
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Some guidelines on building frames for AC
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To be able to enumerate all holdings a preliminary frame is indispensable. It is
made of a set of physical materials (cartographic maps, EA maps, etc.).
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When PHC does not include specific questions on agriculture, the economic
activity status must be considered together with occupation and industry in order
to provide an approximation for households involved in own-account agricultural
production.
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The minimum data items recommended to be included in the PHC to better
identify such households are: (i) Whether the household is engaged in any form of
own-account agriculture production (ii) measure of farm size, e.g. area of land &
the number of livestock.
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Use of farm registers & other statistical/administrative sources to build a
frame.
•
Preparation of sampling frames corresponding to each sample selection stage,
including detailed mapping to support the field data collection (when sampling is
used). The frame requirement is directly related to the type of sampling.
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Using new technology for building frames, e.g.: GPS, GIS, Remote Sensing.
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Frame requirements for various
census modalities (1)
Frame requirement for a classical census
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An initial frame of holdings in the household sector and in the nonhousehold sector is needed.
The total area of the country is unambiguously divided into nonoverlapping subdivisions - EAs in such a way so that the enumeration
work in each area can be handled by a single enumerator during the
census data collection period.
Coordination of the construction of the EAs of the population and
agricultural censuses.
Use of available cartographic materials. It might be necessary (due to lack
of good maps) to prepare sketches for the EAs.
Conduct a listing operation due to lack of reliable information, if needed.
Establish a sampling frame (when sampling is used).
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Frame requirements for various census
modalities (2)
Frame requirement for a modular census
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As for the classical census, an initial frame of holdings in the household sector (or
EAs) and in the non-household sector is needed to conduct the complete
enumeration for the core module (CM).
The CM should provide the necessary information for building appropriate
sampling frame(s) for the subsequent supplementary module(s), SM(s).
A built-in process should be developed for generating sampling frame(s) for SM(s),
according to the design(s) decided.
Frame requirement for an integrated census and survey modality
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As for the classical census, an initial frame is needed for organizing the complete
enumeration for the CM.
The frame requirements for rotating modules are similar to those for SM of the
modular census.
Frame requirement: modality based on use of registers as a
source of AC data
o
The frame requirements for field operation component are identical to those
discussed in the modalities above.
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Use of sampling for census enumeration
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In a classical census sampling may be applied when:
 using the short-long questionnaire concept (the short
questionnaire is administered to all target population of holdings,
while the long questionnaire is administered only to a sample of
such holdings); or
 conducting a sample-based census as a single one-off
operation.
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In the modular approach sampling is needed for selection of
holdings to apply the supplementary module(s).
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When the census is part of an integrated census/survey
modality rotating modules are conducted on a sample basis.
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Use of registers as a source of census data could also be
combined with filed enumeration on a sample basis.
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Sampling techniques
Sampling techniques: simple random sampling (SRS), systematic sampling (SYS),
stratified sampling (STR), sampling with probability proportional to size (PPS),
multivariate probability proportional to size (MPPS), cluster sampling, etc.
One-stage (element) sampling:
 a fairly complete and up-to-date frame is required.
 is not always feasible when conducting ACs, especially in countries with a not well
established system of agricultural surveys.
Cluster sampling:
 a sampling frame at the element level is not needed for the whole population.
 cost efficiency, that is, the relatively low cost per sample element.
 in practice, clusters tend to be internally homogeneous, and this intra-cluster
homogeneity increases standard errors and thus decreases statistical efficiency.
The multi-stage sampling procedure, which involves multiple sampling at different
stages, aimed at achieving maximal accuracy of statistics for allowed costs, is largely
employed for ACs and surveys.
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Main types of sample designs for
census enumeration
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Sample designs can be classified according to the type of
sampling frame used, i.e. based on:
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List frames
Area frames and
Multiple frames.
The main difference between these types of sample designs is
whether the final stage of sample selection is a listing of
holdings or households, or is land based, or if a combination of
a sample of area elements with a list sample is applied to obtain
the census estimates.
Handbook on Master Sampling Frames for Agricultural
Statistics (Global Strategy, 2015).
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List sample designs (1)
•
The most commonly used sampling procedures in ACs.
• List sample designs often include some strata of “special
holdings” that are completely enumerated, or have a high
sampling fraction.
List sample designs for classical censuses
◊ Element sampling (using SRS, SYS, STR, PPS, etc.).
◊ Single-stage cluster.
◊ Multi-stage sampling.
◊ Country examples:
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Croatia : One-stage sample design based on STR
Suriname: Combination of complete enumeration with
cluster sample design
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List sample designs (2)
List sample designs for modular census
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A multistage sampling for supplementary modules:
 Suits very well in-depth surveys when the application of detailed and
time consuming questionnaires is needed.
 It is commonly applied, where EAs are the PSUs and holdings are
the SSUs.
 Depending on the availability of information about PSUs, SSUs,
etc., the use of STR or selection with PPS would be appropriate to
improve the efficiency of the sample design.
Country examples:
 Togo: two-stage sample design for supplementary modules
 Burkina Faso: two-stage probability sample design
 India: combination of one-stage and two-stage sampling designs.
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Sample designs based on area frames and
multiple frames
Sample designs based on area frames
o Well suited for estimating parameters related to land areas such as cropland and can be
used to evaluate the quality of the data collection.
o Has major limitations for estimating other parameters required by a census (such as on
livestock, demographic and social characteristics, etc.):
 Many important agricultural characteristics have a skewed distribution;
 The sampling of area units alone increases the sampling variance. When land of a
large holding belongs to a sampled segment, the expansion factor can lead to
inaccurate estimators for some variables. For instance, farms that have livestock
and do not have their own land are difficult to pick with an area frame.
Sample designs based on multiple frames
o The joint use of area and list frames: a list of relatively few “special holdings”
complement the larger population of smaller farms covered by the area frame.
o Country examples of agricultural censuses conducted in Fiji and Puerto Rico.
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Choice of sample design
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Achieving maximal accuracy for available total budget (resources), or
minimal costs for required accuracy/ level of aggregation for census
data: two ways of stating the aims of efficiency.
Resources include the availability of trained and experienced personnel
in sampling techniques, financial and information resources.
The sample design should be simple enough to operate in the field.
List sample designs versus area sample designs.
Multiple frame designs versus area sample designs.
Multiple frame sample designs versus list sample designs (pros and cons
determined by strengths and weaknesses of the area component of the
former):
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Precision of the estimates.
Basis for a crop yield estimation.
Complexity of implementation.
Mapping requirements.
Proximity of the holder or respondent to the holding.
Distinguishing characteristics.
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Complete enumeration versus sample enumeration census
Complete enumeration
Advantages
1.
2.
3.
4.
Reliable census results for the smallest
administrative and geographic units and on rare
events (such as crops/livestock types)
Provides a reliable frame for the organization of
subsequent regular infra-annual and annual sample
surveys. In terms of frames, it is much less
demanding in respect of the holdings’
characteristics
Requires fewer highly qualified statistical
personnel with expert knowledge of sampling
methods than a census conducted on a sample
basis.This is particularly important in countries
with limited technical expertise
Aggregating data from a complete enumeration is
straightforward and does not involve statistical
estimations.
1.
2.
3.
4.
Sample enumeration
Is generally less costly that a complete
enumeration
Contributes to decrease the overall
response burden
Requires
a
smaller
number
of
enumerators and supervisors than a
census
conducted
by
complete
enumeration. Consequently, the quality
of data can be expected to be much
better because of the employment of
better
trained
enumerators
and
supervisors and better quality control
Require less processing capacity and the
results are usually available sooner.
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Complete enumeration versus sample enumeration census
Complete enumeration
Sample enumeration
Disadvantages
1.
2.
3.
High cost and administrative complexity
High overall response burden
Requires a very large number of field staff. As a
result:
candidates with the desired qualifications
might not be available in the required
number
the standard might be lowered
adequate training of a large number of
field census staff in a short period of time
is also challenging, with a consequent
effect on the quality of data
4.
The amount of data to be processed is very
large. The results may be considerably delayed if not
sufficient data processing capacities are in place.
1.
2.
3.
4.
5.
6.
The amount of subnational data and
cross-tabulations that can be produced
is limited
Cannot provide accurate information on
events that occur infrequently
May not ensure an adequate or
complete
frame
for
subsequent
agricultural surveys
Requires a reliable sampling frame
Requires personnel who are well
trained in sampling methods and analysis
Analyzing the data from a sample
enumeration requires the use of more
complicated techniques.
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Discussion points
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Relevance of:
 Sections on: mapping, frames, sample
designs, advantages and
disadvantages of complete
enumeration and sample enumeration
censuses.
 Boxes and country examples.
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THANK YOU
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