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Disclaimer: Statistical math person I am not...

I'm a programmer that has been tasked with finding the percentile a child will fall in depending on their weight and age according to the CDC.

The CDC site with the data is: http://www.cdc.gov/growthcharts/percentile_data_files.htm

I am using the Weight-for-age data from the CDC and I beleive I can obtain the correct z-score for a child given their weight for age. Now that I have a z-score...what do I do? How can I turn that Z-Score into a percentile? From what I've researched it sounds like I need a standard normal table find the percentile, but can I use any standard normal table I find? How exactly do I get the corresponding percentile for a given Z-Score? I am even on the right track?

EDIT: I would like to get childs specific percentile for their weight if possible. The data tables provided by the cdc give percentages in increments (ie: 3, 5, 10), but I would like to know if a child falls into the 2nd percentile. I just don't know if getting an exact percentile is possible.

AGoodDisplayName
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If you follow the instructions on the page you linked to find the z-score using the LMS parameters, then you can use any standard normal table (they should all be the same) to find the percentile. Since you're programming, you should look to see if your programming language has a "Normal quantile" or "Normal inverse cdf" function (qnorm in R), which does the calculation for you so you don't need a table.

The 3rd, 5th, etc. percentiles that are also included in the files you can download are unnecessary and you can just ignore them. The LMS method will give you an "exact" percentile (up to the error of the distribution fitting, but there's nothing you can do about that--it's baked into the data).

Jonathan Christensen
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  • Yeah I have standard normal table that I was using for some other standard deviation stuff, I was just not sure that they would all be the same. It seems to be giving me the correct results when I compare it to the CDC charts. Thanks for the function tip I was unaware that such a function existed. Unfortuneatly for me I have to use the table as the function is not available in the version of the language I'm using. Since I already have some previous working code I just have to plug in the table thats already in the DB. Thanks again. – AGoodDisplayName Jan 03 '13 at 17:35