Transforming a sample design for taking into
account new statistical needs, new
information or new technological instruments
for data collection
Elisabetta Carfagna
Professor, University of Bologna, Italy
[email protected]
Background and justification
• Various agricultural statistical systems
• New statistical needs
• New low cost technological instruments for data
collection
– GPS
– PDAs
• New information is available (e.g. a land cover
map produced for controlling subsidies)
Alternative strategies
• First strategy: the sample design adopted in
the previous years is preserved
• Second strategy: the sample design is
conceived ex novo
• Third strategy: small changes are applied to
the sample design
Italy, Agrit project
from early 90’s to 2000
• From early 90’s to 2000
– Stratified random sampling of about 5,000
segments with physical boundaries
– Stratification: land cover map from photointerpretation of Landsat TM images
POPOLUS
• Availability of POPOLUS:
– grid of points 500 meters apart
– photo-interpreted according to land cover
legend
– geometrically corrected photos, scale
1:10,000
• POPOLUS first phase sample (used for
stratification)
Agrit project from 2002
• From 2002
• Two phase stratified random sampling of
84,000 un-clustered points
– CVs less than 5% for main crops at national
level
Agrit new technological
instruments
• From 2009 Ultra Mobile Personal
Computer (UMPC) - PDA with GPS and
GPRS/UMTS and Wi-Fi connection
• Data collected immediately sent to the
central acquisition unit
• Change of quality control
Refresh project for control of
declaration
• Availability of new information:
• “Refresh” project for control of declaration
of farmers (2009-2010)
– Photo-interpretation for the whole Italian
territory of:
• geometrically corrected aerial photos
• very high resolution satellite images with
spatial resolution 0,5 m
• POPOLUS update
New information needs
• Baseline indicators for agro-environmental
policy and rural development policy (EU)
– Agro-environmental parameters
– Farmland of high naturalistic value
– Risk of erosion
Observation on small circle around
the point
• Observations on small circle around the
point
– Irrigation
– Grass or bare soil (for permanent crops)
Observations on square segments
• Observations on square segments:
– Irrigation, presence and kind
– Erosion, presence and level
– Bushes
– Small wood lots
– Isolated trees etc.
Pilot project
• Pilot project for:
– Detecting the problems of the ground survey
– Evaluating the variance of the variables
– Computing sample size
– Identifying the optimum segment size
First strategy-preserving the sample
design
• Advantages of preserving the sample
design
– Perfect comparability over time
– Change detection
– Experience of the project managers and of
the enumerators
– The reliability of the data collection
procedures has already been assessed
First strategy-preserving the sample
design
• Disadvantages of preserving the sample
design
– New information (e.g. land cover map) cannot
be used at the design level
– Only some new parameters can be estimated
– Cannot take complete advantage from the
characteristics of new technological
instruments
Second strategy-sample design
conceived ex novo
• Advantages of conceiving the sample
design ex novo
– New statistical needs satisfied
– New available information used at the design
level
– Maximum advantage from new technological
instruments for data collection
Second strategy-sample design
conceived ex novo
• Disadvantages of conceiving the sample
design ex novo
– Not complete comparability
– Difficult change detection
– Project managers and the enumerators have
to adapt to new procedures
– Reliability of the data collection procedures
has to be assessed
– Cost
Third strategy-small changes
• Advantages of small changes
– New statistical needs partially taken into
account
– Comparability over time
– Experience of the project managers and of
the enumerators
– Reliability of the data collection procedures
Third strategy-small changes
• Disadvantages of small changes
– New information (e.g. land cover map) cannot
be used at the design level
– Few new parameters can be estimated
– Partial advantage from new technological
instruments for data collection
Thank you for your attention
[email protected]