What do percentiles mean?
A percentile is the percent who have the same or a lower score; therefore, a percentile shows how your child’s score relates to the scores of other children.
- Standardized Test: A score at the 85th percentile on a standardized test (e.g., Math, Reading, etc.) typically means that your child performed as well or better than 85% of other students (in their grade).
- Behavior Checklist: A score at the 85th percentile on a problem behavior scale typically means that you rated your child as having as many or more problem behaviors than 85% of other children (in their age range).
- IQ: An IQ score is often reported with a corresponding percentile. If a child has an IQ of 100, the corresponding percentile is typically the 50th percentile, meaning that the child’s IQ is the same as or higher than 50% of other children’s IQ.
Intelligence is usually assumed to approximate a normal distribution, and IQ is commonly reported with a mean of 100 and a standard deviation (SD) of 15 (see table on the right). In the standard normal distribution shown below, the green part is usually called the “Average” range, which is within 1 standard deviation of the mean (or average). In a normal distribution, most children (i.e., about two-thirds or 68.2%) score in the “Average” range within the 16th and 84th percentiles.
In a school setting, students who score near the top of the “Average” range (up to the 84th percentile) are generally considered average, usually meaning that they don’t qualify as talented and gifted (TAG). Likewise, student who score near the bottom of the “Average” range (down to the 16th percentile) are generally considered average, usually meaning that there’s no reason to suspect a disability that would need specially designed instruction (SDI) through a individualized education plan (IEP) from Special Education. If your child’s performance is in the “Average” range, it is still your right to request a Special Education eligibility evaluation from your child’s school (see my article on requesting an evaluation), and if you do, it might be helpful to clearly explain why you believe your child has a disability, because your child’s performance is not substantially impaired–it’s average. Although your school might not provide Special Education, teachers often offer struggling students informal supports, sometimes called “tiered” or “Tier 2” or “Tier 3” supports, which might include small-group instruction in class while others are doing independant work, after-school tutoring, etc. If you think that your child is not performing up to their potential, you might also consider other explainations, such as difficulties with attention or focus. Children with a diagnosis, such as ADHD, usually qualify for a Section 504 Plan that can include accommodations. Note that getting a 504 Plan is usually less involved than getting an IEP.
Another way to look at percentiles (in a normal distribution) is lining 50 people up based on their score from lowest to highest, as shown above. Notice the “Average” range, depicted in green. The difference between the bottom and top of the “Average” range is 2 standard deviations and includes 34 people (i.e., about 68%). Now look at the blue range that is above Average. The difference between the bottom and top of the above Average range is also about 2 standard deviations but includes only 8 people (i.e., about 16%). This is what a normal distribution looks like in real life. A normal distribution that might be seen in a classroom of 25 students is shown below.