2.1: Anthropometric Indicators Measurement Guide - Linkages Project

APPENDIX
1.Calculating
Z-scores
9.
Using Distributions by Standard Deviation
The reference population data are
available with the mean measurement
values and the measurement values for up
to +/- 3SD displayed, for the three
nutritional status indices. These values
are given for each month of age up to 119
months, for both boys and girls. The data
may be viewed in the following three
formats: (1) table; (2) graph; and (3)
spreadsheet. An example of how a table of
weight-for-age values for boys can be used
is as follows.
Procedure
A 19-month-old boy who weighs 9.8 kg is
compared to the reference standards. Use a
distribution of the reference standards that
already has values for the standard
deviations calculated. First, the appropriate
distribution should be consulted.
Example: Part of the table below is the
weight-for-age by standard deviation for
boys. Age in months is listed in the far left
column. The mean or expected values for
an “average” healthy boy of each age is
located in the middle column. The
measurement values range from -3 to +3
SDs, with the standard deviation for the
lower and upper halves of the distribution
also shown. The correct line of the table
should be found (lines vary by months of
age or centimeters of length). The child’s
measurement should then be pinpointed
along this line.
Example: In this case, we need to locate
the line for the age 19 months and then
find where 9.8 kg falls. The table shows
that such a child falls between -2 and -1
SD.
Therefore, we would say that a 19 month
old boy who weighed 9.8 kg is between -2
and -1 SD from the mean. To obtain a
more accurate statistic, a Z-score would
have to be calculated.
Calculating Z-scores
When the mean and standard deviation
for a set of data are available, as they are
with the reference standards, a Z-score
can be calculated. In this case, a Z-score
calculated for an individual tells exactly
how many standard deviation units his
measurements are away from the mean of
the reference distribution. A positive Z-
score means that an individual’s
measurements are higher than the
reference mean and a negative Z-score
means that the measurements are lower
than the reference mean. The advantage
of calculating a Z-score is that it provides
more precision than just locating a
position on a table, as we did above. Only
the mean and standard deviation are
needed.
Procedure
Assume we have the same 19 month old
boy from the example above, who weighs
9.8 kilograms. If we look at the reference
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