Setting Up the Frame - Sampling Spatial Units for Agricultural Surveys - page 95

the sampling frame and the actual population at the date of the survey. Spatial
frames, in which geographic elements are sampled instead of farms, provide a good
alternative (FAO 1996 , 1998 ), and satellite images are a major tool to define such
frames (Gallego 1999 ).
Sometimes spatial frames are used to improve an imperfect frame. For example,
a survey of agricultural holdings might use a frame of telephone numbers
supplemented by a spatial frame. The sample drawn from the telephone list will
not cover holdings without telephone service. However, constructing the entire
survey from a spatial frame may be too expensive. Thus some surveys use a spatial
frame to improve a frame with known coverage defects (Kennel 2008 ).
The main advantages of using spatial frames are (Nusser and House 2009 ):

• Complete coverage of the target population . This means that when all the
sampling units are aggregated, the entire population is completely covered and
every sampling unit has a known chance of being selected (Cotter et al. 2010 ).
The sampling units do not overlap, and there are no gaps between adjacent
sampling units. This ensures the unbiasedness of the survey estimates with
measurable and higher (with ps , see Sect. 6.4 ) precision. Complete coverage

is also useful in multiple-frame (list and spatial, see Sect. 10.6 ) surveys where
the spatial frame is used to measure the degree of incompleteness of the list
frame. Conversely, list frames are often incomplete or inaccurate, and they
contain an unknown amount of duplication. This advantage of spatial frames
suggests their use in many cases: if another complete frame is not available and
completeness is not guaranteed by the combined use of different registers, if an
existing list frame changes very rapidly or is outdated, when an existing frame
comes from a census with a low coverage, to verify the rate of coverage of
existing archives on farms and updating them (Carfagna 1998 , 2001 ).
• Timeliness . This is a very important advantage of spatial frame surveys as it
allows generating timely estimates on cultivated areas and on expected pro-
ductions. Generally, agricultural surveys based on a list frame need a very long
time period.
• Longevity . Once a spatial frame is constructed, it remains updated for a long
time. They only need updates for land use changes. The list frames accuracy
degenerates rapidly over time, and they must be continuously and systematically
updated to be really effective. This is expensive, time-consuming, and requires
specialized staff.
• Versatility . Since sampled reporting units can be associated with a portion of
land, a spatial frame can be used to collect data for multiple variables in one
survey, e.g. crop surface, livestock, economic and environmental data. Indeed
spatial frames can also be used for economic-type surveys where defining the
reporting unit for a farm is more difficult (Fecso et al. 1986 ; Vogel 1995 ).
• Objectivity of data collection and non-sampling errors reduced. Spatial frame
sample designs also allow objective estimates of characteristics, such as land
cover/land use and measure of areas, that can be observed on the ground, without
interviews since directly observed by surveyors in the field. Besides,

π

the