Choosing a reference population for the comparison of nutritional status of children has also been a subject of debate. Among growth 'reference standards' available, those of Boston or Harvard, the National Centre for Health Statistics (NCHS), and NCHS-WHO (World Health Organization) are widely used in developing countries. The Gomez Classification used the Harvard standard which was based on anthropometric data from a group of middle-class Caucasian children living in the US during 1930-1939. The data were collected from about 400 children at regular intervals from birth to 13 years. In 1974 the NCHS compiled a growth reference for US children taking two sets of data gathered from a contemporary US child population with different socio-economic and
ethnic backgrounds (Hamill et al., 1979). One set relates to children from birth to three
years collected by the Fels Research Institute. The other was from the Health
Examination Survey (HES) of 1963-1970 and Health and Nutrition Survey (HANES) of 1971-1974. This reference has been widely used in many countries irrespective of their different living standards, ethnic or racial and geographic distributions. The World Health Organization has also used the NCHS data base and prepared a new set of reference tables, including the centile values for growth monitoring purposes (WHO,
1978).
A comparison of Harvard, NCHS and WHO standards by Stephenson and colleagues (1983) showed very close similarities at 50th percentiles. The Harvard standard contains height (or length) values of children measured according to recumbent length while the other two standards used recumbent length up to 24 months and standing height thereafter. Despite these changes in the procedures of measuring,
Stephenson et al. (1983:16) found no major differences between the three growth
standards especially for young children.
The use of a reference standard from a developed country to assess the nutritional status of children in developing countries has raised doubts. The main concern in using an international standard is whether or not child growth is influenced by genetic factors. According to some, there are observable genetic differences in
growth (Graham et al., 1979; Seth, Sundaram and Gupta, 1979). For instance, Graham
and others found, when the Boston standard was used, substantial racial differentials in
age-specific growth rates among poor urban pre-school children in Peru (Graham et al.,
1979:705).
Habicht and others examined the question of ethnic or racial differentials using evidence from studies, representing varied racial, geographic and socio-economic backgrounds, including data from Bogota, Colombia and Black children in the US; they concluded that differences in height and weight are very small between the ethnic groups; 3 per cent and 6 per cent respectively. By contrast they found that the growth
differentials are greater (12 per cent for height and 30 per cent for weight) among pre school children of similar ethnic and geographic backgrounds according to the social class or economic background.
Therefore, they concluded that the differences in growth between any elite ethnic groups are so small that 'height and weight data from any well-to-do children can be used as standard to compare the mean growth curves of pre-school children' (Habicht
et al., 1974:614). In attempting to resolve the problem of a reference population Graitcer and Gentry (1981) examined the studies carried out covering children from privileged classes in Egypt, Togo, and Haiti and compared them with the NCHS-CDC (Center for Disease Control) growth standard. They found that the growth patterns of privileged children in the three countries were very similar to those of children in the reference population. They also observed that at certain heights and ages, privileged children were slightly heavier than the reference children.
Accordingly they stated that there is no difficulty in using 'one reference' for all countries in the assessment of growth patterns of children. Similar results have been
found in other studies as well (Bohdal, 1969; Blanco et al., 1974; Amirhakimi, 1974;
Morley and Woodland, 1979; Mora, 1984:104; Martorell, 1985:24-25; Briones et al.,
1989). Therefore it is suggested that the differences in growth pattern between social groups are mainly a reflection of health and environmental factors rather than genetic factors. Similar evidence is available from Sri Lanka as well: the Sri Lanka Nutritional Survey of 1975/76 found that for both height-for-age and weight-for-age, although the values are below the reference for each age, the median values of growth of the upper socio-economic class pre-school children of Colombo, had a growth pattern very close to the reference population (USDEH, 1976:44).
In 1971 the International Union for the Nutritional Sciences (IUNS) proposed the that creation of growth standards for national populations based on the weight and height data from elite groups or children living in 'optimal' settings (IUNS, 1972). Goldstein and Tanner (1980:584) claimed that such an attempt is fruitless as any
reference population derived from the data collected from developed countries is inappropriate for use in developing countries; they suggested that national growth standards derived from data on elite or high socio-economic groups are equally
inappropriate. Their view was that even for in-country comparisons, one growth
standard would not serve the purpose and there should be several standards for different socio-economic groups.
In view of the growth differentials between reference populations in the developing world, particularly in Asia, Eusebio and Nube (1981:1223) considered that the 'conclusion that ethnic differentials are negligible' was unwarranted, particularly for the Asian populations. They considered the growth values observed among the better- off children in developing countries as a reflection of what they called 'attainable growth’, and suggested that when there were wide differences between population groups, to use local growth standards or Western standards could be used with suitable adjustments to the cut-off points for the identification of the malnourished.
It remains unresolved however, whether height (or length) is as relatively free from the influence of ethnic differentials as weight. It is generally believed that growth patterns of children under the age of five, unlike those at and after the age of puberty, are not very much influenced by their genetic backgrounds (Waterlow, 1980b:717). Martorell, Mendoza and Castillo (1988) who examined differentials in stature among different child population groups of the world found that children of Asian origin, mainly Japanese and Chinese, were shorter than the NCHS children. The difference in stature among children at ages 5-7 months that can be attributed to genetic influences is estimated to be about 3.5 cm. Martorell and others also showed that among the population groups of similar ethnic origin the differentials were much greater: about 12
cm. A large part of the difference was therefore due to socio-economic and
environmental causes (Martorell et al., 1988:64).
An important difference between the growth standards of developed countries arises from differences in the pattem of breastfeeding; longer breastfeeding durations in
developing countries compared with the reference population give rise to faster growth potential in the full breastfeeding period of the first six months or so. To overcome this, some researchers have proposed separate references for the first six months and for other periods (Whitehead and Paul, 1981, 1984); this adjustment seems unnecessary.
In addition, a few developing countries such as Colombia, Brazil and India have attempted to provide their own reference standards for growth monitoring. It is not feasible for each country to have its own reference standards as, inter alia, it is not always possible to have a representative sample of well-fed population and to find a sufficient number of babies (at least 200 at each age) to prepare reliable growth
standards according to the suggested criterion (IUNS, 1972; Waterlow et al., 1977;
WHO, 1978). This is even more difficult in a country where poverty is a serious problem as Martorell (1985) points out: the prevailing low stature among children of different socio-economic groups is mainly a reflection of poverty and health problems and these conditions are uniformly common to every ethnic group. Therefore, it may be difficult to find a suitable number of well-off children from these societies for the preparation of growth standards (Martorell, 1985:25). In the circumstances it is not unrealistic to use reference values based on well-fed children in developed countries for evaluation of growth patterns of children in developing countries (Jelliffe, 1966; Haaga
et al., 1985).
Also, when the general nutritional level of a population is rising, having local reference standards are fruitless and poses difficulties, particularly having several sets of standards for different periods and for different social groups: such efforts will lead to confusion (Hansen, 1984:92). Hence it is convenient and useful to use a reference standard from a developed country whose nutritional levels are 'no longer subject to rise'. This is a good reason to use an international growth standard for comparison (Morley, 1977a: 397).
Even though there are growth differentials between the populations, due largely to the cumulative effects of adverse environmental conditions, the growth standards
derived from Western countries can be used as long-term goals to be reached by future generations (Mora, 1984:102). It has been suggested that international standards should
be treated as reference populations and not as norms or targets (Waterlow et al.„ 1977;
Neumann, 1979). Waterlow and others reviewed the growth reference standards
available in 1977 and found that none of the available growth standards met the minimum criteria (listed above) of an ideal growth reference. They noted however, that among the growth standards they reveiwed, the NCHS standard met most of the criteria
and they therefore suggested its use as an international reference standard (Waterlow et
al., 1977:490).
This reference has since been used in most nutritional surveillance programs in the world. Later, the World Health Organization recommended the use of the published NCHS-WHO growth standard for international use for growth monitoring purposes, as it had details of the centile values (WHO, 1978). This reference standard is considered suitable because of the 'detail, extensiveness and potential universality' (Gueri, Gurney and Justsum, 1980:775). Recognizing the suitability of the NCHS-WHO standard, the DHS global program used these values in comparing the weight and height (length) collected at national DHS surveys. Accordingly the SLDHS recoded data tapes, on which the data set for the present study is based, containing the indices of centiles, medians and standard deviation units for the observed weight and length values of each child derived in comparison with the NCHS-WHO growth reference values.