Indicators of SpLD Series
by Jan Thomson-Long
Blinded by Statistics
“Facts are stubborn, but statistics are more pliable”. ~ Mark Twain
There are many quotes about statistics but I’ve saved some for future blogs as I just get the feeling this won’t be my last foray into this topic.
It was prompted by reading an email to a parent this week where the child’s percentile was quoted as if it was written in stone. I have a number of issues with this approach. Firstly, statistics have to be put in context. Just quoting a figure without referring to what it actually is often leads to confusion and misinterpretation. Percentiles are to do with position within a cohort (a group or class, for example) so the cohort being compared against needs to be understood. Also, many people get percentiles and percentages mixed up.
To clarify, a percentage is a number or ratio that represents a fraction of 100. This is really useful for comparing test scores when the number of questions / marks available wasn’t the same BUT becomes less useful as a measurement when there are a smaller number of marks available. However, in order to really understand that percentage, you need to know something more about the test taken. What was its purpose? Was it being taken after the topic has been taught or before etc?
A percentile is a term that describes how a score compares to other scores from the same set so as to understand a percentile you need information about the set. Who is in it? Is that set statistically sound in terms of an even distribution?
If an individual is at the sixty-fifth percentile – their percentile rank is 65. This means that the student had a test score greater than or equal to 65% of the people in the set. Conversely, 35% of students scored equal to or higher than the individual tested.
As a general rule for diagnostic assessments, we don’t tend to use percentiles because of how easily they are misinterpreted. We usually use standardised scores which I’ll explain another day.
Today, I just wanted to warn about the misunderstanding as I saw it in action to the detriment of the student who is achieving above average but that was not at all the impression received by his parents.