Dear Bindi,

We had a similar discussion previously on how to convert percentile estimates to WHO GS z-score, with the conclusion that it is probably not mathematically possible:
https://www.en-net.org/question/2888.aspx

If you are able to access the raw data, then the z-scores can be calculated.

Best wishes,
Tamsin

Thanks Tamsin.

I don't have the raw data. But I did see this (not in the articles):

"All children with weight less than 80% of the median weight of children with the same height in the reference population, and/or suffering from Oedema, are classified as GAM. The World Health Organization describes Moderate Acute Malnutrition (MAM) as GAM in the 79% - 70% range, and Severe Acute Malnutrition (SAM) as GAM below 70%."

Do you understand that to mean that 79% - 70% would correspond to WHZ -2 to -3, and below 70% would correspond to WHZ< -3?

Thanks again.
bindi

Hi Bindi,

Percentage of the median does not correspond precisely with z-scores as they are calculated differently. See these explanations from LSHTM: http://conflict.lshtm.ac.uk/page_123.htm.

However, in the past, percentage of the median was often used with the cut-offs you mention to admit children aged 6-59 months into acute malnutrition management programmes.

This article might be useful to understand the discrepancy between the measures and how they were used historically: https://www.ennonline.net/fex/1/practical

I hope this helps you a little in interpreting the papers you are reading.

Best wishes,
Tamsin

Hi Bindi

How does percentage of median weight relate to weight-for scores?
There is not a direct translation between percentage of median and Z-scores, but we can use the LMS values given in the WHO tables for WFH to calculate the Z-score for individual length/height and weight measurement. For example, using the WHO WFH table for girls at https://www.who.int/childgrowth/standards/wfh_girls_2_5_zscores.txt we see that the median weight for a girl of height 79cm is 10.0Kg. Eighty percent of 10Kg is 8.0Kg, which is between -3 and -2 Z-scores:

We can use the standard formula to calculate the exact Z-score from a weight that is 80% of the median weight for a girl who is 79cm tall:

Z = (((WEIGHT/M) ^ L) -1) / (L*S)
Z = (((8/10) ^ -0.3833) -1) / (-0.3833*0.08943) = -2.60 Z-scores.

For 80% of median weight, the first term in the equation (Weight/M) will always be 0.8. But the L and S values are different for different heights. This means that at other values for height, a girl with a weight that is 80% of the median weight will have a different Z score.

I plotted Z score values for 80%, 75% and 70% of median weight for girls covered by this table, 2 to 5 years old across heights between 65cm and 120cm. For these girls, 80% of median weight varies around -2.6 to -2.4Z; 75% of median weight varies around -3.4 to -3.1Z; and 70% of median weight varies around -4.3 to -3.9Z.

Hope that's of some help.

All the best

Jay

Many thanks Tamsin and Jay. That was very helpful.
bindi