Bell Curve

May 7, 2022

In many of the tests we, and others, use to establish areas of relative strength and weaknesses the results are presented as percentile scores. 

What is a percentile score? For those who have children and have gone through the process of taking them to the pediatrician, you will be somewhat familiar with this. You may have heard something along the lines of ‘they are at the 58th percentile for height and 37th for weight.’ 

The same thing is happening when we discuss test results as a percentile. 

Let’s pretend we are talking about a student who was born on July 11th, 2005. Let’s also pretend it is now September 26th 2016. To find their chronological age you would subtract their birth date from the current date. 

So

2016 – 9 – 26
2005 – 7 – 11

11 – 2 – 15 is how old they are. So they are 15 days into the 2nd month of being 11 years old. 

Many tests create 6 months groupings of children. Using the example above, that student would fall into the category that spans 

11 years – 0 months and 0 days through 11 years – 5 months and 29 days. 

So a 6 month bracket that would more or less be your peer group. In some scenarios the age range is wider however you are still being compared against children relatively close to you in age. 

Let’s imagine 1000 children who fall within that age range are evaluated using a certain test. 

Whatever outcomes are arrived at most frequently will be the high point on the curve at the 50th percentile. Any score that is stronger  would slide to the right on the curve at various intervals up to the 99th percentile. Any score that is weaker will slide to the left on the curve at the same intervals. 

The average range, the range that reflects that skills are developed to a level where they can function with a level of automaticity, is 25th-75th. 

The further you drop below the 25th percentile, the more likely it is that whatever is being measured will not function in the way you would want and need it to.  

We don’t exist to create students who are Shakespearen when they write or mini Einsteins when they do math. What we do want is for students to be able to interact with their class work without feeling like the cards are stacked against them. That when they put in work they see outcomes on par with the efforts they put in. That they feel successful. In order to see that, you have to have a certain level of automaticity in the skill.  Not every student will see that at the 37th percentile and above, but most do. When you see skills below the 37th percentile, and on intake our students are all well below the 37th percentile, you may see no ability in the area being measured at all, or some ability that is very inefficient. This will lead to students trying to get the work done, a huge effort, with poor outcomes, both in the work and emotionally. 

Short version on percentiles: if you score at the 25 percentile, roughly 75 percent, or 75 out of a hundred students the same age as you, were able to do the work at a higher level than you were. 

If you score at the 1st percentile, 99 percent of children in your peer group are stronger in that area than you are.