Z score ppt

7/30/2019 Z score ppt

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Group 4 Pediatrics

Z score


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Three different systems by which a

child or a group of children can be

compared to the reference population: Z-scores (standard deviation scores),

Percentiles,

Percent of median.

For population-based assessmentincluding surveys andnutritional surveillancethe Z-score is widely recognized asthe best system for analysis and presentation ofanthropometric data

Z-score is the most appropriate descriptor of malnutrition


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Weight-for-height, height-for-age and weight-for-ageare interpreted by using theZ-score classificationsystem.

The Z-score systemexpresses theanthropometric value as anumber of standard

deviations or Z-scoresbelow or above thereference mean or medianvalue.


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A fixed Z-score interval implies a fixed height or

weight difference for children of a given age.

The formula for calculating the Z-score is:

Z-score (or SD-score) = (observed value - median value of the referencepopulation) / standard deviation value of reference population


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Cut-off points and summary statistics

For population-basedassessment, there are twoways of expressing childgrowth survey results using

Z-scores. One is the commonly used

cut-off-based prevalence;

The other includes thesummary statistics of the Z-

scores: mean, standarddeviation, standard error,and frequency distribution.


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Prevalence-based reporting: Prevalence-based data are commonly reported using a cut-off value, often +2 Z-scores.

The WHO Global Database on Child Growth and Malnutrition uses a Z-score

Cut-off point of =15


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Summary statistics of the Z-scores:

The mean Z-score, describes the nutritional status of the entire populationdirectly without resorting to a subset of individuals below a set cut-off.

A mean Z-score significantly lower than zerothe expected value for thereference distributionusually means that the entire distribution hasshifted downward, suggesting that most, individuals have been affected.

Using the mean Z-score as an index of severity for health and nutritionproblems results in increased awareness that, if a condition is severe, anintervention is required for the entire community, not just those who areclassified as "malnourished" by the cut-off criteria

The observed SD value of the Z-score distribution is very useful forassessing data quality.

With accurate age assessment and anthropometric measurements, theSDs of the observed height-for-age, weight-for-age, and weight-for-heightZ-score distributions


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Summary statistics of the Z-scores:

An SD that is significantly lowerthan 0.9 describes a distributionthat is more homogenous, or onethat has a narrower spread,compared to the distribution ofthe reference population.

If the surveyed standarddeviation of the Z-score rangesbetween 1.1 and 1.2, thedistribution of the sample has awider spread than the reference.

Any standard deviation of the Z-

scores above 1.3 suggestsinaccurate data due tomeasurement error or incorrectage reporting.


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The observed SD value of the Z-scoredistribution is very useful for assessing dataquality. The expected ranges of standard

deviations of the Z-score distributions for thethree anthropometric indicators are asfollows:

height-for-age Z-score: 1.10 to 1.30

weight-for-age Z-score: 1.00 to 1.20

weight-for-height Z-score: 0.85 to 1.10

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Z score ppt