I asked a similar question with regard to this issue about two weeks ago. The formula that you are referencing is used to calculate percentiles from grouped data. You can find a reference to this on page 26, formula 3.3 or 3.4. Glass & Hopkins, Statistical methods in psychology and education, 3rd edition (1996). I hope that helps.
Bob Intrieri
Here is the formula that we are working with:
L + [ (N) (P) - nl ] i
nw
Where L represents the lower real limit of the category containing the percentile of interest.
N is the total number of scores in the distribution
nl is the number of individuals with scores less than L
nw is the number of individuals with scores within the category containing the percentile of interest
i represents the interval of the category that contains the percentile of interest.
Given the i portion of the formula, shouldn't that formula yield the same percentile regardless of whether the scores have been grouped or not? I've consulted several undergraduate statistics books that I have laying around, but these offer either examples of grouped data or examples of ungrouped data using what appears to me to be the same formula. Am I missing something here? This problem has not come up before though I admit that I've been only teaching statistics for the past two years. Frankly, I'm a bit embarrassed to bring it up to the entire group because I am pretty sure that either I am making a simple mistake somewhere or I am overlooking something that is overly obvious.
I'm returning these homeworks tomorrow and I'd love to have a good answer for them.
One other quick "I should have known-type" question. Do cranial nerves have the same extent of contralateral control of the face and head as the primary motor cortex does for the rest of the body?
As always, your answers would be greatly appreciated.
Robert C. Intrieri, PhD
Department of Psychology
Western Illinois University
1 University Circle
Macomb IL 61455-1390
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