3.1 Sampling Design
The sampling design of the Labor Force Survey adopts that of the Integrated Survey of Households (ISH) which uses the new master sample design starting July 1996. The multi-stage sampling design of the master sample consists of 3,421 PSU’s in the expanded sample for provincial level estimates with a sub-sample of 2,219 PSU’s designated as the core master sample for regional level estimates. The July 1996 Labor Force Survey was based on the expanded master sample.
3.1.1 Domains
The domain for the new master sample is similar to that of the previous ISH design with an addition of 23 newly created domains. The urban and rural areas of cities and municipalities with a population of 150,000 or more are considered as separate domains. The other urban and rural areas in each of the 77 provinces are likewise treated as separate domains. In view of the creation of ARMM and the separation of Marawi City and Cotabato City from Lanao del Sur and Maguindanao, respectively, the urban and rural areas of the two cities also form separate domains.
3.1.2 Sampling Frame
The frame for the first and second stages of sample selection were based mainly on the results of the 1995 POPCEN. The 1995 POPCEN list of barangays with the household and population counts is used in the first stage of sample selection. The stratification of barangays included in the frame, however, are based on the 1990 CPH and other administrative reports from the field offices of NSO.
The enumeration areas which constitute the secondary stage sampling units are those that were formed during the 1995 POPCEN.
Isolated barangays and/or barangays that are difficult and expensive to reach are excluded from the sampling frame. However, critical areas or barangays with peace and order problem which is generally temporary in nature are included in the frame.
The frame for the third stage of sample selection is the list of households from the 1995 POPCEN.
3.1.3 Stratification Scheme
Barangays in each domain were explicity stratified by urbanity. Within the urban/rural barangay stratum in each of the provincial domains, the barangays were implicitly stratified by municipal district, serpentine ordering of the municipalities, and grouping of the barangay, based on accessibility and more or less similarity in socio-economic characteristics and religious background of the population. The implicit stratification ensured geographic coverage.
3.1.4 Sample Selection
The multi-stage sampling design of the master sample involves the selection of the sample barangays for the first stage, selection of sample enumeration areas or EAs for the second stage, and the selection of sample households for the third stage in each stratum for every domain.
The sample barangays were selected systematically with probability proportional to size from the list of barangays that were implicitly stratified. The preliminary count of households based on the results of the 1995 POPCEN was used.
The selection of sample EAs for the second stage was also done systematically with probability proportional to size. The EAs are those that were formed during the 1995 POPCEN. An EA is a physical delineated portion of the barangay. For barangays that were not divided into EAs, the barangay was treated as an EA.
The selection of sample households for the third stage was done systematically from the 1995 POPCEN List of Households.
3.1.5 Sample Size
The new master sample consists of an expanded sample of 3, 421 sample barangays (2,045 urban and 1,376 rural ) to improve the precision of provincial-level estimates from the household surveys. A subsample of 2,219 sample barangays (1,330 urban and 926 rural) which has about 27,000 households is defined as the core master sample which will provide reliable survey estimates at the regional level as well as for the larger provinces.
3.1.6 Estimation Procedure
A. Calculation of Final Weights
The basic weight for the sample households in the July 1996 LFS can be expressed as in inverse of the probability of selection, as follows:
Nh Nhi Nhij Nh
Whij = --------------- x --------------- x --------------- = ----------------
bh x Nhi Nhij nhij bhx nhij
where:
bh = number of sample EA’s selected in stratum h (domain city,other urban or rural, within province) for the expanded master sample
Nhi = number of households from the 1995 POPCEN frame in the i-th sample barangay in stratum h
Nh = total number of households from the 1995 POPCEN frame (cumulated measure size) fro stratum h
Nhij = number of households from the 1995 POPCEN frame for the j-th sample EA in the i-th sample barangay in stratum h
nhij = number of sample households selected in the j-th sample EA in the i-th sample barangay in stratum h for the July 1996 LFS (fixed at 12)
Since the sample households for the July 1996 LFS were selected directly from the 1995 POPCEN frame, the basic weight is the same for all households within the stratum (that is, the sample is self-weighting within stratum). However, after the basic weight is adjusted for nonresponse, the weights may vary slightly by sample EA within stratum.
The basic weights need to be adjusted to take into account the sample EA’s which were not enumerated, as well as the household non-interviews, followed by an adjustment based on the population projections for the domain. Therefore the weight adjustment will be carried out in three stages for the LFS data.
(1) Weight Adjustment Factor for Sample EA’S Which Were Not Enumerated
The first stage weight adjustment will be carried out at the stratum level (domain city, other urban or rural, within province). This adjustment factor (Aid) will be based on the number of sample EA’s selected in the stratum (bah’) divided by the number of sample EA’s actually enumerated:
bh
Aih = -------
bh'
Using this adjustment factor is equivalent to substituting bhwith bh' in the formula for the weight specified earlier, as if bh' sample EA’s had been selected in stratum h instead of bh'.
(2) Weight Adjustment Factor for Noninterview Households
In adjusting the weights for noninterview households, it is necessary to distinguish valid non-interviews (refusals, not at home) from selected units which are out-of-scope (vacant or demolished housing units), and whether or not they were replaced. In the case of households which could not be located, it will be assumed that they are valid households for estimation purposes; however, in future surveys the interviewers should make a strong effort to determine the status of such households. For the July 1996 LFS, the interviewers were instructed to replace any sample household which could not be located with a household from the list of selected replacement households which they were given. However, they were not always able to replace such households. Also, in a few cases the enumerators replaced households which refused to be interviewed, were not at home, or the housing unit was vacant.
The second stage weight adjustment factor for non-interview households will be carried out at the level of the sample EA. This adjustment factor can be defined as follows:
nhij'
A2hij = ------------
nhij''
n'hij = number of valid households selected for the survey in the j-th sample EA in the ith sample barangay in stratum h (excluding demolished or vacant housing units); for the July 1996 LFS, this would be equal to 12 minus the number of sample households which were not interviewed.
n''hij = number of households with completed survey interviews in the j-th sample EA of the i-th sample barangay in stratum h (that is, n’hij minus the number of valid noninterview households which were not replaced); for the July 1996 LFS, this would be equal to 12 minus the number of sample households which were not interviewed.
(3) Weight Adjustment Factor Based on Population Projection
In order to adjust the survey estimates of total population for each domain to reflect changes in the population over time, a final adjustment factor can be applied at the domain level, based on the best population projections for the corresponding reference period. It should be noted that this adjustment should only be carried out for higher-level domains for which good population projections are available. The population projections at the national and regional levels should be fairly Lreliable. However, the reliability of demographic projections tends to decrease for lower levels of disaggregation, since updated information on the differential fertility, mortality and migration rates may not be available for lower-level geographic areas. For this reason the weights for the 1995 FPS were only adjusted at the regional level. Although the LFS used population projections at the provincial and city levels in the past, the NSO should verify the reliability of theses projections before deciding to continue using this estimation methodology. Otherwise, it is recommended to adjust the weights at the regional level.
Before the final adjustment of the weights based on population projections, it is necessary to carry out a preliminary adjustment of the weights to take into account the non-enumerated EA’s and noninterview households. In the case of the LFS, the weights would be adjusted as follows:
Whij' =WhijxAlhxA2hij
As described in the “Weights and Estimation Procedures” specifications, theses preliminary weights will be used to calculate from the survey data the weighted number of persons in each domain (15 years or older, and less than 15 years). For each domain and age group, the final weight adjustment factor would be calculated as follows:
Xpdg
Adg = ------------
Xdg'
where:
Adg = last stage weight adjustment factor for the sample survey records for persons in age group g for geographic domain d
Xpdg = projected number of persons in age group g for geographic domain d
Xdg'= preliminary weighted estimate of the total number of persons in age group g for geographic domain d, from the LFS data
3.1.7 Geographic Area Coverage
For most part, statistics have been limited to the socio-economic data at the national level. It is very evident though that there is the need for information at the local level. In this regard, the LFS sample design has been drawn in such a way that accurate lower level classification would be possible.
3.2 Questionnaire Design
The items of information presented in this report were derived from a structured questionnaire covering demographic and economic characteristics of individuals. Refer to Appendix C for detailed information on the items included.
3.3 Method of Collection
Personal interview is deemed most applicable for the LFS owing to the complexity of the questionnaire, the details required, and the level of education of respondent in sample households.
NSO Statistical Coordination Officers/Assistants (SCOs/ASCOs) and Statistical Researchers served as interviewers during the operations. Supervision and monitoring of survey operations were done by the Regional Administrators/Provincial Statistics Officers of NSO, most of whom have experience and have undergone training on various types of surveys and censuses.
3.4 Data Processing
Data processing involves two stages: manual processing and machine processing. Manual processing refers to the manual editing and coding of questionnaires. This is done prior to machine processing which entails code validation, consistency checks as well as tabulation.
Enumeration is a very complex operation and oftentimes it happens that accomplished questionnaires have some omissions and implausible or inconsistent entries. Editing is meant to correct these errors. For purposes of operational convenience, field editing is usually done. The interviewers are required to review the entries at the end of each interview. Blank items which are applicable to the respondents are verified and filled out. Before being transmitted to the central office, all questionnaires are edited in the field offices.
Coding, the transformation of information from the questionnaire to machine readable form, is likewise done in the field offices.
Machine processing involves all operations that are done with the use of a computer and/or its accessories, that is, from data encoding to tabulation. Coded data are usually in such media as tapes and diskettes.
Machine editing is preferred to ensure correctness of encoded information. Except for sample completeness check and verification of geographic identification which are the responsibility of the subject matter division, some imputations and corrections of entries are done mechanically.
Further machine processing for this round, is done at the central office of the National Statistics Office.