The longitudinal mixed model
5.1 Preliminary analysis
5.1.1 Classification of the height and weight measurements
Identification of t h e model
by the WHO. The reference population chosen by NCHS was a statistically valid random population of healthy infants and children. The NCHS/WHO international reference tables have been used for standardizing anthropometric data around the world and can be found in the Food and Nutrition Technical Assistance (FANTA) website: www.fantaproject.org/publications/anthropom.shtml and the NCHS website:
www.cdc.gov/nchs/about/major/nhanes/growthcharts. Anthropometry is the study of the technique of taking body measurements especially to assess and predict perfor- mance, health and survival of individuals and reflect the economic and social well being of the population.
These reference standards are used to standardize a child's measurement by com- paring the child's measurement with the median or average measure for children of the same age and sex. For example, if the height of a nine year old boy is 130 cm, it is difficult to know if this is reflective of a healthy nine year old boy when there is no comparison to a reference standard. The reference or median height for a population of nine year old boys is 133.7 cm and the simple comparison of heights would conclude that the 130 cm tall child is almost 4 cm shorter than expected.
However the 4 cm height difference with the reference for a nine year old boy is not the same as a 4 cm height difference with the reference for a seventeen year old boy because of their relatively different body sizes due to their difference in age. Taking the age and sex into consideration, differences in measurements can be expressed in a number of ways namely (1) standard deviation units or Z-scores; (2) percentage of the median and (3) percentiles.
The Z-Score or standard deviation (SD) unit is defined as the difference between the value for an individual and the median value of the reference population with the same age or height as the individual, divided by the standard deviation of the values
of the reference population. This is,
. Observed value — median value of reference population Standard deviation of reference population
A positive Z-score means that the individual's measurement is higher than the reference median whereas a negative Z-score means that the individual's measurement is lower than the reference median. The distribution of Z-scores is a normal (bell-shaped or Gaussian) distribution (Cogill, 2003).
The percentage of the median is defined as the ratio of a measured or observed value of the individual to the median value of the reference population with the same age or height for the specific sex as the individual, expressed as a percentage. That is,
~ , ,. Observed value , . . Percentage of median = —— • —; ;—:— x 100.
Median value of reference population
If a child's measurement is exactly the same as the median of the reference population, then the child is said to be '100 of the median'. Lastly, the percentile is the rank position of an individual relative to the reference distribution, stated in terms of the percentage of the group the individual equals or exceeds.
Cut-offs are then used to enable the different individual measurements to be con- verted into prevalence statistics. Cut-offs are also used for identifying those children suffering from or at a higher risk of adverse outcomes. The most commonly used cut-off with Z-scores is -2 standard deviations, irrespective of the indicator used. This means that children with a Z-score below -2 SDs are considered to be moderately or severely malnourished for example children that are underweight (low weight-for-age), that are stunting (low height-for-age) or wasting (low weight-for-height).
The low weight-for-age index identifies the condition of being underweight for a specific age while a deficit in height-for-age is referred to as stunting. A low weight-for- height value helps to identify children suffering from current or acute undernutrition or wasting and is useful when exact ages are difficult to determine.
The cut-off points for different malnutrition classification systems are listed in Table 5.1. The most widely used system is the WHO classification (Z-scores). The Road- to-Health (RTH) system is typically seen in clinic-based growth-monitoring systems.
The Gomez system was widely used in the 1960s and 1970s, but is only used in a few countries now. Mild, moderate and severe are different in each of the classification systems listed in Table 5.1. It is important that the same system is used to analyse and to present the data. The RTH and Gomez classification systems typically use weight-for-age.
Table 5.1: Malnutrition Classification Systems
System Cut-off Malnutrition classification WHO
RTH
GOMEZ
< -1 to > -2 Z-score
< -2 to > -3 Z-score
< -3 Z-score
> 80% of median 60% - < 80% of median
< 60% of median
> 90% of median 75% - < 90% of median 60% • < 75% of median
< 60% of median
mild moderate severe
normal
mild-to-moderate severe
normal mild moderate severe
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For this study, the Z-scores are used to classify the Weight and Height measure- ments. The Weight variable is coded as follows: 1 for Risk of underweight (< -1.25 Z-score); 2 for Normal (-1.25 < Z-score < 1.25); and 3 for Risk of overweight (> 1.25 Z-score). The Height variable is coded as follows: 1 for Short (< -1.25 Z-score); 2 for Normal (-1.25 < Z-score < 1.25); and 3 for Tall (> 1.25 Z-score)