Characteristics of list frames

The directory of units that constitutes the list frame should ideally exhibit at least the following characteristics: a. The list frame must contain well-defined units

The first requisite is that the units are precisely defined. Ambiguous definitions of holdings or parcels can lead to lists that are inadequate for survey purposes.

b. Each unit in the frame should have a unique identifier

The names of administrative units, such as villages, are not necessarily unique within a country. To ensure uniqueness and to facilitate sample selection and control operations, a carefully designed system of numerical identifiers (coding system) is necessary. The list frame contains a record for each frame unit. The only absolutely indispensable item is a unique identifier for each unit. If a unit is selected, the identifier provides the means of access to the population element for performing the survey or, eventually, subsequent sampling operations. The numerical identifiers (primary identifiers) will of course be linked with other identifiers (secondary identifiers), such as place names or coordinates of holdings sites, either within the frame itself or on maps or other auxiliary materials. The primary identifiers are used to select samples;they may also be used to link area units in a frame to the maps and sketches. For all purposes, the use of a hierarchical system is advisable: the first group of digits identifies the highest-level administrative division in which the frame unit is located; the next group identifies the second-level administrative subdivision, and so on, down to the individual frame units. Some provision for distinguishing urban and rural units may also be helpful. Secondary identifiers are used primarily to aid in locating frame units. Typical secondary identifiers are: names, address, coordinates (if GPS was previously used), and instructions on how to locate the unit. For agricultural holdings, the holder’s name is the key identifier. When building the frame from a field operation such as a census, the full name and alias (if any) should be recorded. It is very important to record the holding address as precisely defined in the enumeration manuals. For example:

ADMINISTRATIVE DIVISION PRIMARY IDENTIFIER

(code) SECONDARY IDENTIFIER

Region R Department/State/Province R D District/Municipality R D M Community/Settlement R D M C CENSUS DIVISION Enumeration Area R D M C E HOLDING

Holding number in the census cartography R D M C E H Names and addresses of holding (if any), and

holder or coordinates of main buildings or main parcel.

Not coded, onlyfor identification in field work

c. Completeness

The third aspect refers to completeness. The concept of “completeness” involves two aspects: coverage and information provided for each frame unit. For frames that cover 100 percent of the target population, completeness of coverage can be verified by cumulating measures of size (such as land area of the census holdings) of the frame units and comparing the totals for administrative areas, such as states, provinces or districts, against available measures taken independently (such as cadastral registers, maps or satellite images). For frames containing a list of names or other units that are not necessarily related to land areas, checking for completeness is more complicated. Several lists can be used for comparison: farmers’ association lists, administrative registers at field level, etc. Frame completeness also requires that specific items of information, such as stratification variables and measures of size, be included in the record for each frame unit.

d. Rules of association

Clear rules of association must be established between frame units, the target population, the associated reporting unit, and the data for the items of interest to be collected. The rules of association ensure that the probability of selecting every hectare of a given crop, every animal, every source of income, etc. is known and based on the probability of selecting the sample unit. In other words, the basic requirement of a rule of association is that it must assign to every sampling unit and item of interest a non-zero probability of selection that can be accurately determined. However, the association rules linking items of interest with frame units may not be clear. For example, the frame may list three units that appear as three different holdings. However, these may actually be three parcels of a unique holding. For the same reason, holdings operated in partnership by several persons also pose problems for list frames. It is common for the same holding to appear with different names in different directories or lists, and it is sometimes difficult to determine the identity. Therefore, it is not unusual for list frames to contain duplications.

e. Frames must be up-to date

List frames easily become obsolete. Populations are dynamic: some holdings disappear and new ones replace them. Holding operators change, new holdings appear, and urban expansion and new infrastructure result as farm land disappears, etc. In Section 5.8, the issue of maintaining and updating list frames is analysed.

f. Existence of auxiliary information

The final requisite is that frames must contain auxiliary information to improve the sampling designs and estimators. The frame should include accurate and up-to-date supplemental data for each frame unit. Measures of size, such as total area, number of household members or number of land parcels are especially useful for stratification or sampling designs based on PPS selection methods. These auxiliary variables are necessary when using ratio or regression estimators. They are also important in the planning stage of the survey, to distribute the workload among interviewers. Errors in supplemental data do not necessarily lead to biases in survey results. They can, however, limit the efficiencies that can be gained from using better supplemental data for sample design or estimation. In other words, sampling errors are increased.

5.3. MAIN ISSUES ARISING FROM THE USE OF LIST FRAMES TO BUILD