Download - 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