Expert: Ahmed Salami - 7/17/2010

Question
How do you calculate a normal curve equivalent score when all you have is a percentile score?  For example the percentile score is 33

Answer
Hi Casey,
A percentile is the value of a variable below which a certain percent of observations fall. For example, the 10th percentile is the value (or score) below which 10 percent of the observations may be found.
On a normal curve, the percentile represents the area sum from left to right. We can then say that the normal score corresponding to the 33rd percentile is the score where the area to its left is 33% or simply 0.33
Now, from tables we have
z = -0.44

Regards