The "iles" give a general picture of how your score compared with that of other test takers.
(It is comparable to "percents", but don't say that)
 

Percentile "ranks"
 -scores of students are arranged in rank order from lowest to highest
 -the scores are divided into 100 equally sized groups or bands
 -the lowest score is "in the 1st percentile" (there is no 0 percentile rank)
 -the highest score is "in the 99th  percentile"
   (you can't score in the 100th percentile because you can't beat your own score)
 

If you "scored in the 66th percentile", you scored "as well as or better than" 66% of the group.

If your score was the same as "the mean" for that test, you scored in the 50th percentile.(The mean is the middle of a distribution, and the 50th percentile is the same...the middle of the group.)

If 700 people took a test, how many people would be in each percentile band?
   (Divide 700 by 100 bands/ranks)

If only one student is taking the diagnostic achievement test, how do we determine his/her percentile ranking?  Who do we compare him/her against?  (The norm groups...kids who have taken the test previously... provided by the test)
 

Deciles ("Deca" means "ten")
 -scores of students are arranged in rank order from lowest to highest
 -the scores are divided into 10 equally sized groups or bands (instead of 100 as with percentiles)
  (Or divide the percentiles into bands of 10 percentile ranks)
 -the lowest score is "in the 1st decile"
 -the highest score is "in the 10th decile"

This "ile" is the most difficult to visualize and state.  For example, the 62nd percentile rank falls in the 7th decile rank (remember to add "1" to the tens column of the percentile rank to get the decile rank)  (The first decile includes the first to 9.99th percentile, so that's why you add a "1" to the tens column to find the decile)

The 4th deciles goes up to the 39.9th percentile.  The 40th percentile start the 4th decile.

Quartiles
 -scores of students are arranged in rank order from lowest to highest
 -the scores are divided into 4 equally sized groups or bands
  (Or divide the percentiles into bands of 25 percentile ranks)
 -the lowest score is "in the 1st quartile"
 -the highest score is "in the 4th quartile"

1st quartile is 1st to 24.9th percentile
2nd quartile is 25th to 49.9th percentile
etc.

Tom McIntyre at www.BehaviorAdvisor.com