Growth can be studied by two methods
- cross-sectional method and
- longitudinal method.
To overcome the shortcoming of both these methods, sometimes, growth may be studied
- linked-longitudinal method and
- mixed-longitudinal methods.
In cross-sectional research, the subjects are measured only once. Subjects of different ages are included in a cross-sectional study to provide a general description of age-related growth changes. In other words, the method of study using different children at each age is called cross-sectional. It is basically a cross section of a given age group, sex group or of a given population.
The method of study using the same child at each age is called longitudinal method. In a longitudinal study each individual is measured over a number of years. In other words, the same subject or group of subjects is measured repeatedly from year to year. A birth to maturity study may go upwards to 20 years to complete. Details of individual development are provided by this approach.
Longitudinal studies are costly and dependent upon the continuous cooperation of the subjects. It is therefore essential that the highest standards of accuracy should be maintained in collecting and recording the data and that the methods of analysis used should be those appropriate to the methods of study, yielding the maximum and most accurate information concerning individual growth.
In a comparative survey of children’s growth in different populations, we are more concerned with the means and variations of groups of children than with the growth patterns of individuals.
Generally, therefore, the information used is from cross-sectional studies. Cross-sectional methods are adequate for studying distributions of various measurements in different individuals at different ages and for constructing standards of growth attained, e.g., height and weight standards. In these circumstances the relative ease and rapidity with which results may be obtained from a large number of cases makes the cross-sectional methods preferable to the longitudinal one. Cross-sectional methods are also obligatory in circumstances where continuity is not possible, e.g., autopsy studies on internal organs .
From a cross-sectional study we can calculate the centiles of the distribution. The centiles
from cross-sectional studies are often used as the basis for population standards. A much larger number of subjects are necessary for creating effective standards. The numbers of children measured in each age group should be approximately proportional to the rate at which growth is taking place.
A serial study in which a group of children is followed such that some children leave the study and others join it as new entrants at different ages, giving various degrees of longitudinally is termed as mixed longitudinal study. Thus, such a study combines data for individuals who were measured on all occasions and data for individuals who were measured on only several occasions.
Merits- Longitudinal studies
- provides information about the distance (gross size) growth attainments, they also provide growth velocity related data i.e., about individual rate of growth measured by increment between two successive periods (e.g., kg/year or cm/year)
- provides timing of particular phases like ‘onset of juyenile growth spurt’ or ‘adolescent growth spurt’ of individuals.
- these studies help to understand the influence of genetic and environmental factors on the growth dynamics of children.
- Longitudinal studies provide opportunity to have information about the duration of any disease or nutritional stress with which any child might have remained afflicted with for a longer period.
- Effect of intervention (medical/nutritional etc.) may also be assessed with the help of these studies as children included in such surveys are often monitored periodically.
Demerits – Longitudinal studies
- Longitudinal studies are very expensive, require great skills to organize and require a well-organized logistical team.
- Studies are very laborious and time consuming, these studies require patience, perseverance and motivation on the part of both subjects as well as researchers who undertake it.
- The researchers have to stick to the already planned periodic schedule during the entire duration of the study, which at times becomes difficult to adhere to because of certain compelling circumstances.
- The number of subjects who complete a longitudinal study tends to be small. It is not always possible to maintain consistency of sample size throughout the entire span of the study.
- Hawthrone Effect: The principal drawbacks of longitudinal studies are the time these take to complete and the small number of subjects that can usually be followed-up. One of the ways to maintain adequate sample size in such studies is to generate a high level of involvement amongst children and their parents in the study. Unfortunately, greater the involvement the more likely to affect the growth of children participating in the study. The phenomenon of greater involvement of parents in a longitudinal series yields to Hawthrone Effect, which results from subjects knowing that they are a ‘part of study. Continuous and persistent health care advice to the parents (whose children are part o fa longitudinal study) may become aware of relationship between growth and need for proper environment. As a result they may change their children’s diet etc. which may lead to improved general growth status of the children included in the longitudinal study.
Merits- Cross sectional studies
- They are cheaper, less time consuming, can include much larger number of subjects in a brief duration and provide important information about the distance attainments or gross size attained by children during a span of time.
- Results of cross-sectional studies give information on growth, maturity, performance or physical activity status of a sample of children and the variability within the sample.
- Such studies provide information on body size, stage of maturity or level of performance and especially activity attained at the time of survey.
- This method is very useful in constructing growth standards for communities.
- Cross-sectional surveys are valuable in assessing the nutritional status and health related problems of children prevailing in different communities at any given point of time.
Demerits of cross sectional studies
- They can never reveal individual differences in rate of growth (ie. growth velocity) of different body dimensions of children, since in this method we measure each child only once without any periodic follow up.
- These individual differences which reflect the cumulative effect of various genetic, environmental, hormonal, nutritional, psychological, and socio-cultural factors on human growth.
- Cross-sectional data do not provide precise information about timings of particular phases of growth like onset of “Juvenile growth spurt”, attainment of “Peak height velocity”, “Peak weight velocity” etc.
- They tell us nothing about variability around the mean.
- They tell us nothing about individual increments from one year to the next, i.e, about individual rates of growth.
Merits- mixed longitudinal studies
- They are relatively cheaper to conduct and also less time and effort consuming as compared to pure longitudinal studies.
- These studies also provide us with both distance and velocity curves, i.e. these include information on both growth status and rate of growth.
Demerits of the mixed longitudinal studies
- The estimation of growt velocity of different body parameters from mixed longitudinal data involving missing values is a tedious task and special statistical methods are required to get relevant information out of such data.
- In some circumstances the manipulation of increments derived from each individual measured twice or more is reasonably efficient and simpler. The means of such increments may be used to calculate more efficient measurement-at-given age or distance means at successive ages.