Category Archives: Biostatistics

Understanding Growth Charts

Disclaimer: This article is primarily intended for my students. However, others may also find it useful.

Background information:

Growth: The regular increase in size and/or weight of a living being.

Development: The increase in skills and functions of a living being.

Reference Values: Values used for national and international comparisons. These values may have been generated by observing a variety of individuals, often from different racial and socio-economic backgrounds so that they may be generalizable to many populations. However, the characteristics of a particular population may be much more uniform/ homogeneous than the reference population. In such instances, the use of standard values is recommended.

Why do we need reference values?
Just as reference units for length and weight were developed to assist with trade, we need reference values to accurately determine the health status of children in a given country. This enables international comparisons; locates children’s growth with respect to their healthy peers; enables identification of malnutrition; permits prioritization of interventions and funding; is useful for evaluation of interventions targeting malnutrition, etc.  Simply put, without reference values, one wouldn’t know if a particular child’s growth was similar to, or different from what is expected for a given age and sex at a point in time.

Standard values: Values used for local (National) comparisons. These values are  generated by studying individuals from the country/ state in question, and may therefore reflect local characteristics more accurately than reference values. However, such values are unsuitable for international comparisons as they don’t reflect the diversity seen between various racial, socio-economic and ethnic groups.

Can we exclusively use local standards for growth monitoring? Yes, and No- although such usage would find favour with those arguing against use of reference values, the inability to make international comparisons would result in international pressure to use reference values. Several international agencies monitor children’s growth on a global level. They would be unable to monitor such growth unless all nations used the same values to assess children’s growth. The reports of such agencies form the basis for intervention and funding, too, both nationally and internationally.

Percentile: A statistical measure in which a population is divided into 100 equal parts, with each part representing 1% of the total.

Percentile versus percentage- what’s the difference?
Percentiles are relative measures, while percentages are absolute measures.

Example: An individual appears for two examinations, one which uses percentiles to determine who passed, while the other uses percentages for the same purpose. Both have the same numerical value- 50th percentile; and 50% as the cutoff, but although she scored 60/100, she failed the first exam, and passed the second.

This happened because the first exam was very easy, and most candidates scored 70 to 90 out of 100. The cutoff was set at the 50th percentile (50% of scores fall below that value (median)). Since the exam median was around 70, she did not pass.

On the other hand, the relative performance of other candidates did not matter in the second exam- all that she needed to pass was a score equal to or above 50%.

Z-score: A statistical measure that is obtained by standardizing values obtained from a survey. It helps locate any value in a standard normal distribution by indicating the distance of that value from the mean in terms of Standard Deviations. Please read the previous article to learn about z-scores and standard normal distribution.

Key Messages:

A growth chart is an example of appropriate technology- it is a tool that visually depicts the growth of a child from birth to 5 years.

There are separate growth charts for boys (blue border) and girls (pink border).

Each growth chart has weight in kilograms on the y-axis, and age in months on the x-axis.

Previously, the growth chart used in India was based on the NCHS (Harvard) standards, and the curves represented percentile values.

The current ICDS growth chart uses the 2006 WHO child growth standards,  and uses z-scores for weight-for-age. This is because weight is the most sensitive indicator of growth; and while weight may decrease, height will not.

While the previous standards showed how children grow, the current standards show how children should grow (given optimum nutrition, healthcare and environmental) regardless of ethnicity, socio-economic status and type of feeding.

If one plotted the weight for age of children using the previous standards, then did the same using the new standards, one will notice

  • considerable increase in rates of underweight in the first half of infancy that decreases later
  • much higher rates of wasting (low weight-for-age) during infancy

using the new WHO standards.

The ICDS growth chart has three curves:

slide_30

  • the top curve represents +2 SD from the mean
  • the second curve represents -2 SD from the mean
  • the third curve represents -3 SD from the mean

Why these curves?

From the standard normal distribution, we know that 95% of all values are located between -2 SD and +2 SD of the mean. This range is considered the range of normal values.

normalstandard

The area between -2 SD and -3 SD contains around 2.2% of values. This zone indicates underweight.

The area below -3 SD of the mean contains around 0.1% of all values, and values falling here are considered to reflect severe underweight.

Growth Monitoring

Weighing of the child at regular intervals, the plotting of that weight on a graph (growth chart) enabling one to see changes in weight, and giving advice to the mother based on this weight change is called ‘GROWTH MONITORING’.

It is the change in weight over a period of time which is most important, rather than the weight itself, and should be done frequently:

  • birth- 1 month weekly,
  • one month- 3 years every month and
  • 3-5 years at every three months.

However, children who are severely underweight, or who have not gained weight for 2 months, or who are “at risk” of under nutrition, should be weighed frequently- preferably every month.

Useful Links:

Link to NIPCCD’s Growth Monitoring Manual (English) (PDF):

http://nipccd.nic.in/elearn/manual/egm.pdf

Link to WHO page ‘Child Growth Standards’:

http://www.who.int/childgrowth/en/

 

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