Most definitions of migration use space and time criteria (Young, 1980: 112). Census type definitions usually measure internal migration as a move across an administrative boundary during the migration interval (Khoo and Rowland, 1985: 167; Goldscheider, 1971). The migration interval is usually the intercensal period in which case the migration is fixed-period. It may also be the period between birth of each person and the census date in which case it is a lifetime migration (Shryock et al, 1976).
The 1980 CPH uses both the fixed-period and the lifetime migration definitions of migration. Fixed-period migration in the 1980 CPH is defined as a change in usual place of residence during the intercensal period 1975-80. Thus, a fixed-period migrant is one whose place of usual residence in 1980 differed from that in 1975. A lifetime migrant is one whose place of usual residence in 1980 differed from
her place of birth. The place of usual residence could be a city/municipality, a province or region, depending upon the type of migration made. The question was 'In what city/municipality and province did fname of person) reside on May 1, 1975?'
Fixed-period migration in the 1980 CPH may be classified according to the boundary crossed - city or municipality, province or region. A person whose municipality of residence in 1980 is the same as that in 1975 is considered a nonmigrant, more specifically, an intra-provincial nonmigrant. Those whose municipality of residence in 1980 differed from that in 1975, where both municipalities were located in the same province, were classified as intra-provincial migrants. Persons whose province of residence in 1980 differed from that in 1975 are called inter provincial migrants. If the provinces were located in the same region, the migrants are appropriately called intra-regional migrants. However, if the provinces of residence belonged to two different regions the migrants are called inter-regional migrants. Under this typology, the migrants are collectively called inter-boundary migrants in this study.
During the enumeration, a special code 88 was assigned if the city or municipality of residence in 1975 was the same as that in the 1980 Census. Code 89 was also used if the city or municipality of residence in 1975 was different from that in 1980 but both belong to the same province. If the province of residence in 1975 was different from that in 1980, the appropriate geographic code was assigned. This coding scheme was adopted not only to facilitate coding and data entry but also to minimise coding and data-entry errors. However, unless there was very close supervision, enumerators might have haphazardly entered either of those codes instead of writing the name of the municipality and province of residence in 1975 in cases where the province in 1975 differed from that in 1980. This could then lead to an underreporting of inter provincial migrants and even internal migration in general.
In terms of the level of boundary crossed, an intra-provincial migrant might have crossed the shortest distance. This is not necessarily true in cases where the
municipality of residence was on the opposite side or end of a province compared with one crossing provincial boundaries but where the municipalities of residence in 1975 and 1980 shared common provincial boundaries. The typography of the place also affects the concept of physical distance. Thus, interpretation of results pertaining to distance is affected by this limitation which arises from the definition imposed. Using the level of boundary crossed as a measure of distance, an inter-regional migration would mean the longest distance migration while intra-regional migration is intermediate between the intra-provincial and inter-regional migrations. Again, the difficulty in classifying migrants in terms of an absolute space holds. Provinces may share the same boundaries as the regions, thus, migration between two contiguous provinces belonging to two different regions means shorter distance to traverse than migration from one island province to a neighbouring island province in the same region. On the other hand, Zelinsky (1971) noted that with the development of transportation and communications, the functional space has shrunk which makes distance less significant in defining migration. However, it may be argued that for a developing country such as the Philippines, affordability may be more important than availability and accessibility of transportation and communication facilities. With the country's topography it takes a specific-area analysis to determine the implication of distance in defining migration.
While data on urban or rural residence at the time of enumeration were available in the 1980 Census, this information was not obtained for usual residence at the previous date (1975). Thus, analysis on movement between urban and rural areas from 1975 to 1980 is not possible. Refer to Appendix B for definition of urban and rural areas.
Lack of a standardised definition of migration often leads to difficulty in qualifying moves which are too short a duration and/or distance as 'migration' (Goldstein, 1983: 12). The kinds of errors inherent in census-type migration are often mentioned in the literature. (For example, see Shryock et al, 1976; Khoo and Rowland, 1985: 167; Zelinsky, 1971: 226.) In the 1980 CPH, at most only one move was counted
for each person during the migration period; intervening moves made during the period were not counted (Hugo, 1991: 10). Migrants who died during the period were omitted in the classification altogether. The omission of multiple moves, however, may be partly offset by the inclusion of return migrants (Khoo and Rowland, 1985: 168-172) or those who changed residence during the period but returned to their residence at the beginning of the period and were counted as nonmigrants. Seasonal migrants may also be included because of the difficulty in identifying them during the data collection (Khoo and Rowland, 1985: 168). This is particularly true in large cities and also in places where hired agricultural labour is common. For example, Cabigon (1990: 67) observed that the level of registration of death events by place of occurrence was overreported in Metro Manila. She attributed this to the concentration of the best medical facilities in Metro Manila so that migrants who came to the metropolis for medical attention could have died there.
During data collection information on a person's previous places of residence was given by the respondent for the other members of the household, especially when that member of the household was not present at the time of the interview. Misclassification could arise from this procedure not only because of recall problems but also because of the respondent's lack of knowledge of the other household member's place of previous residence (Hugo, 1991: 9). Proxy respondents may also fail 'to report persons who have left for non-economic, personal or idiosyncratic reasons and ... these may be more likely to be females' (Bilsborrow, 1991: 8).
Most studies dealing with data limitations tended to assume that the limitations apply to both males and females alike. This may not be the case as the following causes for the underestimation of female migration which Bilsborrow (1991) described do not necessarily apply to males. One cause is the data collection procedure adopted by data collection agencies in classifying the type of move. 'Methods of data collection that do not collect information on migration over short distances tend to omit females differentially' (Bilsborrow, 1991: 14). However, lack of data on short-term migration does not generally underestimate female migration, particularly in the Philippines
where males predominate in seasonal labour migration. On the other hand, the distinction between short-term and long-term migration largely depends on the respondent's concept of usual residence. Thus, conclusions drawn from studies dealing with motives of female migration may be biased because short-term migration is generally associated with seasonal employment while long-term migration could be related to other reasons (Bilsborrow, 1991: 5-21).
Another cause of bias comes from the type of activities women engage in (Bilsborrow, 1991: 16-17). Socially desirable activities tend to be exaggerated and socially undesirable ones, omitted. This kind of bias is likely to affect the present data. While in other countries, refusal or non-responses may be large for women who are engaged in low-status occupations such as prostitution and domestic helpers and also for housewives who are too preoccupied with child-minding and housework, refusals or non-responses are relatively high among the more affluent group. Either way, this would lead not only to an undercount of migrants but also to underestimation of female economic activity and the importance of economic motivation in migration decisions. However, analysis of trends implied by census-based migration is not likely to be affected. Another source of bias but which may not be relevant in this country is the images of women's role in society. This is particularly true in societies where women's role is confined in the home and work outside the home is not socially acceptable. To the extent that women actually work without the knowledge of the male relatives, their economic activity will be understated (Bilsborrow, 1991: 16-17).
Underreporting of female migration and economic activity could also arise from the choice of respondents. Studies have shown that male respondents are likely to underreport female out-migration and economic activity. Self-reporting is likely to be more accurate and complete (Bilsborrow, 1991: 22; Hugo, 1991: 3). In the Philippines, the respondent is more likely to be a female rather than male because housewives and domestic helpers are usually the ones left at home.