Now, I'm having what I think is an analogous situation with a statistics homework assignment. I had given students a set of scores for them to organize into a frequency distribution and to calculate various statistics, including percentiles. Yesterday when I received their homeworks and began to check them I found that most students organized their data into ungrouped frequency distributions as shown in the textbook. A couple of students decided to organize them into grouped frequency distributions with intervals of 50. Hey, no problem there. However, when I looked at their answers for percentiles. Each group was coming up with different answers. Last night I spent over two hours going over their calculations and they appeared to have followed the formula correctly. I woke up this morning thinking that perhaps I had activated a sufficient number of subconscious problem-solving structures that would allow me to discover the answer to this problem, but after nearly an hour at this I think I am ready for someone to point out to me the equivalent of not having turned up the volume.
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.
Miguel
Miguel Roig, Ph.D.
Associate Professor of Psychology
Notre Dame Division of St. John's College
St. John's University
300 Howard Avenue
Staten Island, New York 10301
Voice: (718) 390-4513
Fax: (718) 390-4347
E-mail: [EMAIL PROTECTED]
Home: [EMAIL PROTECTED]
Http://facpub.stjohns.edu/~roigm
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