Posted by
Ornelas, Fermin on
Apr 05, 2007; 5:05pm
URL: http://spssx-discussion.165.s1.nabble.com/Boxplot-seemingly-does-not-show-outlier-tp1074926p1074931.html
Let me take another shot at this. It is not clear what you are trying to
do in your analysis. Having only 4 data points is not a very meaningful
way to conduct statistical research. In most practical statistical
classes you will be reminded of questionable results when you have a
small sample size. None of the properties usually referred in regression
can be verified (normality, constant variance, outliers, independence).
That is what I was referring indirectly when I said "why bother if you
only have 4 observations".
Fermin Ornelas, Ph.D.
Management Analyst III, AZ DES
Tel: (602) 542-5639
E-mail:
[hidden email]
-----Original Message-----
From: SPSSX(r) Discussion [mailto:
[hidden email]] On Behalf Of
Tom Werner
Sent: Thursday, April 05, 2007 7:38 AM
To:
[hidden email]
Subject: Boxplot (seemingly) does not show outlier
In SPSS (12.0 for Windows, Student version) when I attempt to produce a
boxplot of four data points (62, 61, 59, and 33), SPSS generates the
boxplot...
...but does NOT show 33 as an outlier (even though 33 would seem to be
an
outlier relative to 62, 61, and 59 to the casual observer).
(I'm analyzing the scores of sets of four judges and would like to use
SPSS
to produce boxplots to indicate 'outlier' judge scores.)
Even if I change the value of 33 to 13, it still does not show in a
boxplot
as an outlier.
If I add a fifth data point (with a value as low as 50), 33 shows in a
boxplot as an outlier.
Can anyone explain this?
1. Is it because of the even number of data points (four), thus
requiring
that the median be a calculated value?
2. Are five data points that much more powerful than four data points
at
producing a tighter intraquartile range (i.e., a tighter box in the
boxplot), and thus generating an outlier?
3. Is this perhaps a quirk of SPSS?
Much thanks for any help!
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