Understanding Percentiles

Posted on 03 22, 2006 under The Manual by Noodad | Comments

When you have kids you are forced to do math. Somethings are easy. When you wife tells you to put 3 diapers in the diaper bag for instance. Some are a tad harder—124 Pampers baby Dry Size 2 for $29.99 versus 114 Huggies Supremes Size 2 for $24.99. But for me, the hardest thing to figure out is the darn child percentiles when my kids get checkups. I can figure out Earned Run Averages. I can even somewhat figure out QB Ratings. But for some reason I can"t figure out the logic behind percentiles.

Your child"s pediatrician will tell you at every checkup what percentile your child is in for weight and height. Apparently, from birth we are conditioned to compare our children with other children because the percentile figure is based on how your child"s measurements compare to every other child the same age. For instance, if your 6 month old child is 50% in weight it means that approximately half the population of 6 month old children is heavier than your kid and half are lighter than your kid. So in essence, your kid is average.But is that right? They say that America has a weight problem. So if that is true, does that hold true to babies as well? And if that is true, does that mean that average weight is really overweight? They sure don"t use percentiles when it comes to adults. They just say you are underweight or overweight. Sure they will call it moderately abuse but it really is a nice way of saying you are fat.

So what do you want your kid"s percentile to be at? The answer is don"t get hung up with the percentiles. It is used more as a comparison over a long period of time. For instance if your kid is 25% in weight at 6 months and 50% in weight at a year then he caught himself up in those 6 months to an average weight. Of course if at a year your kid is at 95% then you pushed too hard with the Similac. The only time you should worry is when the percentile moves dramatically. But your pediatrician will already be tracking that.

I started stressing when my daughter came up at a high percentile for weight early on. She eventually thinned out as her height developed. Now she is at an appropriate level. Everything is fine now and everything was fine then but these damn percentiles mess with your head. Consult with the pediatrician and make sure your kid is healthy and developing normally and then don"t sweat it.

Don"t sweat it Noodads.

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    The numbers can really screw with your head, if you let them. With the first doctor visits I was clueless. My favorite question, \"what does that mean?\" As time progressed I just wanted to know if they were healthy. I realized that I knew what and when they should be eating. I also knew if they were getting enough exercise. As long as the doctor agrees, I\'m cool.
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    With percentiles in a normal curve, what you really have to look out for is the extremes. The numbers near the 50th percentile do not represent a great deal of difference (e.g., there is not much difference between the real value represented by the 51st vs. the 52nd percentile). \"Normal range\" extends from the 20th to 80th percentile (roughly). Once you get to the ends of the curve, the differences between percentiles represent large differences in the represented values.

    One way to think about it: Every pitcher in professional baseball is in the 99th percentile of pitching (of the world\'s population), but there\'s a huge difference in ability between the best and worst.
 

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