6. SIMULACIÓN Y RESULTADOS
6.3.3. CIRCUITO DE CONTROL DE GENERACIÓN (CCG)
1.418. Depending on the types of problems to be tackled, a country may con- sider applying sampling methods in one or more of the following phases of a popula- tion census: tests of census procedures, data collection for (usually) a subset of topics in addition to those for which universal coverage is required, post-enumeration field checks, quality assurance of data-processing, advance tabulation of selected topics, and final processing and tabulation. Each phase is discussed below.
1. Tests of census procedures
1.417. Planning the various phases of a census often involves choosing among several alternative procedures. Tests conducted on a sample basis provide the best means of determining which alternative to use. The results of such tests facilitate a more desirable allocation of available census resources than is possible otherwise.
1.420. The nature and extent of census testing depend on the information that is available from previous censuses or other sources. If, for example, prior housing statistics are lacking in a country, a pilot survey may be called for to assess in advance the practical problems that will be involved in including specific housing topics in the census.
1.421. When carrying out census tests, probability samples are not usually necessary. Since the purpose of most census pilot and pre-tests is to judge the opera- tional feasibility of a proposed course of action for the main census rather than make population estimates, purposive samples can usually be used for such tests. Purposive selection of one or a few geographical areas is generally preferable for such feasibil- ity testing. Purposive samples are also particularly useful when it is necessary to test census questionnaires and methods in areas with particularly difficult conditions. On the other hand, when overall quantitative measures are needed for comparing efficien- cies of different procedures (for instance, in examining the anticipated response errors arising from different systems of enumeration), random sampling procedures must be used.
2. Enumeration of topics in addition to those for which universal
coverage is required
1.422. The expanded needs in most countries for extensive and reliable demo- graphic data have made the use of sampling a cost-effective part of census taking. Sam- pling is increasingly being used to broaden the scope of the census through the asking of a number of questions of only a sample of the population and households. This use of sampling makes it feasible to obtain urgently needed data of acceptable precision when factors of timing and cost would make it impractical to obtain such data on a complete-count basis.
1.423. It is important to bear in mind, however, that national legal require- ments may make it mandatory to collect certain information on a complete-count basis. Legislation in many countries prescribes complete population enumerations at particular times or makes certain political or administrative dispositions dependent on particular results from a complete enumeration. For example, the apportionment of seats in the legislature among the civil divisions of a country often depends on the number of persons actually enumerated in each division. The data needed for this and similar purposes may not be collected by sampling.
1.424. Census information that is collected for only a sample of the population and/or housing units is usually obtained by one of two different methods. The first pre-designates a systematic subset of census households to receive a so-called long form, or the census form that contains the detailed questions on all topics. Depending on the sample requirements which, in turn, take account of considerations of cost and precision, the systematic subset that is designated for the long form may represent, for example, 1 in 4, or 1 in 5, or 1 in 10 of the census households. Under such a sampling scheme, all other households in the census will receive a short form containing only those questions intended for universal coverage. If countries choose this option, it is recommended that the pre-designation of the sample households that are to receive the long form be carried out at a central location by supervisory statistical staff, since it has been shown that when the enumerators themselves actually identify the sample households the results are often biased.
1.425. The second method of sampling often used involves designating a sample of enumeration areas to receive the long form. In this approach, all households in the
designated enumeration areas receive the long form and all households in the remain- ing enumeration areas receive the short form. The advantage of the first method over the second is that the sampling precision of results is greater in the former because clustering effects increase the sampling variance when whole enumeration areas are used as sampling units. On the other hand, the advantage of the second method is that different enumerator staffs may be trained more easily, since one set of enumerators can be trained only for the long form and the other set only for the short form.
1.426. It is important to make certain that asking questions that are not asked of all persons does not give rise to legal, administrative or even political issues, since census information is required under statute and often with penalty for refusal.
1.427. The suitability of particular questions for a sample enumeration depends on the precision with which results are needed for small areas, and small population groups, and on the enumeration costs involved.
3. Post-enumeration surveys and field checks
1.428. As discussed in the section on the evaluation of census results, it is uni- versally recognized that census taking is not perfect and that errors can and do occur. A highly useful method of evaluating the census results discussed in that section is the use of post-enumeration evaluation surveys (PES). An independent quality check such as a PES can be critical in validating the census count. Whenever a PES is utilized for census evaluation, it is important of course that the design of the PES be based upon sound probability sampling methods.
1.429. The sample design for a PES must duly take account of the measure- ment objectives of the evaluation study. These usually include the need to estimate census undercoverage with a certain degree of reliability. In addition, the estimates of undercoverage may be wanted for geographical areas such as provinces or States, and large cities, for urban rural comparisons and so forth. Such requirements also greatly affect the sample design of a PES, as the necessary sample size is increased substan- tially when estimates of subnational coverage (or undercoverage) are wanted. When designing a PES it is important that:
(a) Time between the census and the PES be minimized to avoid as much recall error as possible;
(b) The PES must be independent of the census. PES interviewers must not have census information about the areas they are working. When inter- viewers have knowledge of census responses, they tend only to confirm what the census recorded;
(c) To preserve the independence of the PES, its data collection and process- ing operations must be completely separate from the census data collection and processing;
(d) Dual system estimation should be used because it requires assuming that the PES is only a second independent enumeration, and not that it is a higher quality (or perfect) enumeration than the census;
(e) The members of households interviewed in the PES should be matched to the census on a case-by-case basis to determine whether they were enu- merated in the census;
(f) The PES must have a rule for people who move between Census Day and the survey interview. For example, the independent sample may be the peo- ple who are residents of the sample areas on Census Day, which includes the out-movers.
4. Quality assurance and improvement programmes
1.430. As mentioned earlier, sampling can be used effectively for measuring and controlling the quality of many phases of census operations (see paras. 1.228– 1.277). The quality assurance measures start with pre-enumeration (designing ques- tionnaires and pilot tests), and continue through enumeration and post-enumeration. Under post-enumeration this includes, in particular, the editing and coding of ques- tionnaires, data entry and tabulation. Even in a country of medium population size, these operations involve millions of questionnaires.
1.431. Every effort should be made to keep operational features as simple as possible. In general, a systematic pattern of selection with random starts is preferable to a random pattern. Measures of quality must be adaptable to simple record-keeping systems.
5. Advance tabulation of selected topics
1.432. A complete national census is a huge undertaking and several months, or even years, may elapse before some of the tabulations are published. It is therefore natural that some countries, particularly those with very large populations, should consider advance, provisional tabulations as a way to ensure that key data are available and are disseminated in a timely manner. Sampling can be availed to serve this need in countries that decide to prepare advance tabulations.
1.433. Preparing advance tabulations through sampling has certain disadvan- tages, however. For the final results to be given, the results tabulated for the sample units have to be integrated with those tabulated for the non-sample units. These opera- tions may increase the total tabulation time of the census and its cost. Precautions are necessary in order to minimize the delay that may be caused in the preparation of the final results. Moreover, issues concerning the differences between the advance tabulations (which are estimates based on a sample) and the final tabulations (which for some topics may be complete counts, while for others estimates based on the long- form sample) must be resolved to the satisfaction, and with regard to the comprehen- sion, of users. Finally, the need for an extensive set of advanced tabulations has been reduced in recent years because the widespread use of microcomputers has reduced the time that was being taken to process the census in many countries. In these cir- cumstances, advanced tabulations programmes are likely to be needed only by very large countries that anticipate extended data-processing operations.
1.434. If sampling has been used as an integral part of a complete enumera- tion to secure information for a subset of topics, as described above, the same sample of units (persons, households or enumeration areas) can also provide a sample for advance tabulations of the census proper. Such a sampling scheme, if it is devised effi- ciently, with a view to securing additional census information by small administrative units, may offer excellent opportunities for conveniently obtaining advance tabula- tions for the same administrative units.
1.435. Even when no sampling has been used in the actual enumeration, a sam- ple design for advance tabulations may be comparatively simple to achieve because the complete census returns provide a sampling frame which can then be used to select the sample for the advance results.
6. Final processing and tabulation
1.436. The principal limitations of complete processing and tabulation of all the information collected in a population census and/or housing census are the length
of time it takes and the costs. Consequently, a country may decide that CSPro and IMPS should be used as processing and tabulation programmes that provide complete tabulation of a set of core items, such as those on the short form (for countries that use sampling for long-form items), while certain other characteristics are processed and tabulated only on a sample basis. In addition, countries should keep in touch with the latest technology to be used for data processing and tabulation.
1.437. In considering the advisability of using sampling in connection with the final processing operations, the following considerations may also be taken into account. There are certain population and housing characteristics about which infor- mation is needed only by large areas and for the country as a whole. Sampling makes it possible to obtain detailed tabulations for large areas, with reasonably small sampling errors, at a much reduced cost and in a shorter time than that needed for tabulations on a complete basis. However, since one of the purposes of a census is to serve local interests, the feasibility of sampling is determined to some extent by the size of the smallest localities for which separate tabulations can be reliably produced.