# Who growth charts and Z-scores?



## Mama8 (Mar 6, 2006)

Does anybody know what a z-score is and how it translate to percentiles? Is there the new WHO growth chart with percentiles instead of z-scores?
Thanks


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## Kleine Hexe (Dec 2, 2001)

I've seen the new charts and they don't make sense to me.







:


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## spughy (Jun 28, 2005)

(Dredging out her rusty statistics knowledge)...

A Z-score is a measurement of deviation from the norm. Its units are standard deviations, so a Z-score of +2 means that the measurement falls two standard deviations away from the norm.

In plain english (and slightly inaccurately) a standard deviation is the average amount that all the measurements deviate from the average of all the measurement (aka the norm). So if your baby's weight is in the range of a Z-score of -1, it just means that your baby's weight is below the norm, but kind of just in an average way, if that makes sense. Typically anything within 2 standard deviations of the norm in most scales is fine and nothing to be concerned about, IIRC.

Because most measurements of any human trait fall into a normal distribution (a bell curve), z-scores are a good way to "eyeball" where a given measurement falls.


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## annekevdbroek (Jun 5, 2005)

Just to add a little more statistics...

Assuming the population curve is normally distributed (any large sample, like a children's growth chart will be) and bell shaped....

68% of the population will fall between +1 and -1 standard deviation of the average score. 95% will fall between +2 and -2 standard deviations of the score.

So - if you have a z-score you can estimate your percentile very roughly this way

-3z = 0.1% percentile
-2z = 2.5nd percentile
-1 z = 15th percentile
0 z = 50th percentile
+1z = 84th percentile
+2z = 98th percentile
+3z = 99.9 percentile

There is a formula for calculating the exact percentile if you have the z score and the standard deviation - but I can't remember the formula adn don't have the time to either work it out with pen and pencil or dig out one of my old stats books. If you do need an exact number feel free to PM me and I'll try to work it out for you.


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## kalirush (Jun 14, 2005)

there are who charts with percentiles on them though just look down a little further

julia (nak)


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