On Wed, 31 Jan 2001 21:47:22 -0600 "G. Marc Turner" <[EMAIL PROTECTED]>
wrote:
> I need some advice on how to handle an issue in my stats course...
>
> I'm currently using the Essentials of Statistics (by Gravetter & Wallnau)
> as the textbook. In the chapter on variability, it uses the real limits for
> the calculation of the range, inter-quartile range, etc. This is actually
> how I learned to do the calculations by hand, and haven't really thought
> much about it until now.
>
> I'm also teaching the students how to use SPSS (v9) to get the same
> information after going over the hand-calculations. BUT, SPSS does not use
> the real limits when calculating the range, quartiles, etc. (For example,
> for one data set we get a range of 9 and an interquartile range of 1.89 by
> hand. SPSS calculates the range as 8 and the interquartile range as 2.)
>
> So, tomorrow afternoon when I go over this in class, and the students ask
> me to explain why the scores are different, what should the answer be? My
> immediate reaction is to say that we shouldn't trust the values of SPSS,
> and this points out why it is important to know how to do the calculations
> by hand. However, I'm not sure how far that will get me in convincing the
> students.
>
Marc:
Definitely, I would not teach them that SPSS is "wrong" in this case.
Instead I would turn this into a discussion of group or class interval
size. When someone reports that his weight is 165 lbs then there is an
implicit assumption that he has been truthful if his weight turns out
to be 164.9 or 165.2. We usually count 164.51 -- 165.49 as a 165. In
other words, we use the mean of the interval to represent the
entire interval. That is what SPSS is doing, using the mean of the
interval.
Depending on circumstances, you may want to compute a range from true
upper and lower bounds or you may want to compute the range from mean
to mean of the intervals.
The important point, and this applies to all stat packages and stat
formulae, is that the person must choose what are the appropriate and
meaningful math operations for the task at hand. This issue is not
solved by choosing to do the compuations by hand or by keyboard.
Ken
PS--Systat uses the same calculation rule as SPSS. Are they both wrong?
----------------------
Kenneth M. Steele, Ph.D. [EMAIL PROTECTED]
Dept. of Psychology
Appalachian State University
Boone, NC 28608
