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Minitab's outliers
Posted:
Sep 21, 1996 11:12 AM
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I may need to correct part of my earlier message. Minitab computes
quartiles as I said. I thought the versions I've used in the past
used these to construct a boxplot but documentation for recent
editions says they use Tukey hinges.
----- Forwarded message from hayden -----
I am not familiar with this particular version of Minitab, but
traditionally Minitab has used the 1.5XIQR rule for detecting
outliers. However, it computes the IQR by subtracting the first
quartile from the third. Despite what they say, this is not what
Moore and McCabe do. Use "describe" on your data and see if you and
Minitab agree on Q1 and Q3.
Minitab approaches quartiles using a general approach that fits all
kinds of quantiles -- deciles, percentiles, etc. In this system, Q1
has rank 0.25(n+1) and Q3 has rank 0.75(n+1). You can see that this
could involve interpolating one-fourth or three-fourths of the way
between adjacent data values, while this could never happen with the
procedure used in Moore and McCabe.
When Tukey invented the boxplot he used approximate quartiles which he
called "hinges". He wanted simple techniques that could be
implemented quickly without any computing machinery. Minitab could
use these hinges and get boxplots exactly like Tukey's, or it could
take the position that hinges are only approximations to quartiles,
and so quartiles are the thing to use and Tukey's handmade boxplots
are only approximations to the "correct" ones Minitab produces. They
seem to have taken the latter position.
----- End of forwarded message from hayden -----
_
| | Robert W. Hayden
| | Department of Mathematics
/ | Plymouth State College
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