[19252] in s-news-athena
[S] One more point on References for late discovery of hole in the ozone layer due to "outlier" deletion
daemon@ATHENA.MIT.EDU (STEVEN.P.MILLARD)
Fri Sep 17 13:59:55 1999
Message-Id: <01BF00FA.71DB88A0.STEVEN.P.MILLARD@prodigy.net>
From: "STEVEN.P.MILLARD" <STEVEN.P.MILLARD@prodigy.net>
Reply-To: "smillard@probstatinfo.com" <smillard@probstatinfo.com>
To: "S-news (E-mail)" <s-news@wubios.wustl.edu>
Date: Fri, 17 Sep 1999 10:30:36 -0700
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Hello Everyone,
Scott Chasalow actually read in depth some of the material posted on
one of the web sites regarding the late discovery of the ozone hole
above Antarctica due to "discarding outliers". According to this
source (see information below my signature), observations less than 180
DU were not automatically deleted but instead were "flagged". So
instead of never having a chance to see the low values, it was then up
to the people who dealt with these data to decide whether to include
these flagged observations in their analyses or not. I'm not sure
whether this makes the people who handled the Nimbus-7 data look better
or worse (of course I always do the right thing with my statistical
analyses and have never make [sic] a mistake), but the point is that
someone, somewhere decided to ignore these "outliers."
Sincerely,
--Steve M.
_____________
| *** | Steven P. Millard, Ph.D.
| * |
| * * * | P robability, TEL: 206-528-4877
| * * * | S tatistics & FAX: 206-528-4802
| * | I nformation E-mail: SMillard@ProbStatInfo.com
| * | Web: www.ProbStatInfo.com
| *** | 7723 44th Avenue NE
|___________| Seattle, WA 98115-5117 USA
-----Original Message-----
From: CHASALOW, SCOTT [AG/2165] [SMTP:SCOTT.CHASALOW@cereon.com]
Sent: Thursday, September 16, 1999 12:10 PM
To: 'smillard@probstatinfo.com'
Subject: RE: [S] References for late discovery of hole in the ozone
layer due to "outlier" deletion
Hi Steve,
Thanks for this interesting discussion. It might be worth noting the
NASA defense on this issue, which is somewhat more than pretending it
never happened. If you go from the site you mention,
http://curriculum.qed.qld.gov.au/kla/eda/ozone.htm
to
http://www.nas.nasa.gov/Services/Education/Resources/TeacherWork/Ozo
ne/Ozone
_History.html
you find the following, written by one Brien Sparling (whoever that
is):
QUOTE:
In 1974 M.J.Molina and F.S.Rowland published a laboratory study
demonstrating the ability of CFC's to catalytically breakdown Ozone in
the presence of high frequency UV light. Further studies estimated that
the ozone layer would be depleted by CFC's by about 7% within 60yrs and
based on such studies the US banned CFC's in aerosol sprays in 1978.
Slowly various nations agreed to ban CFC's in aerosols but industry
fought the banning of valuable CFC's in other applications. A large
shock was needed to motivate the world to get serious about phasing out
CFC's and that shock came in a 1985 field study by Farman, Gardinar and
Shanklin. Published in _Nature_, May 1985, the study summarized data
that had been collected by the British Antartic Survey showing that
ozone levels had dropped to 10% below normal January levels for
Antarctica. The authors had been somewhat hesitant about publishing
because Nimbus-7 satellite data had shown no such drop during the
Antarctic spring. But NASA soon discovered that the spring-time "ozone
hole" had been covered up by a computer-program desiged to discard
sudden, large drops in ozone concentrations as "errors". The Nimbus-7
data was rerun without the filter-program and evidence of the
Ozone-hole was seen as far back as 1976.
It is important to note that the description of the discovery of the
ozone
hole presented above is not accurate. Dr Richard D. McPeters,
Principal
Investigator, Earth Probe TOMS at Goddard Space Flight Center explains
"Ozone is derived on a FOV-FOV basis and there has never been a filter
applied as described. The explanation I usually see is that we "threw
out"
the low values. This is not correct either. Our software had flags for
ozone
that was lower than 180 DU, a value lower than had ever been reliably
reported prior to 1983. In 1984, before publication of the Farman
paper, we noticed a sudden increase in low value from October of 1983.
We had decided that the values were real and submitted a paper to the
conference the following summer when Joe's paper came out, showing the
same thing.
As the first one in print, he gets full credit for discovery of the
ozone
hole. It makes a great story to talk about how NASA "missed" the ozone
hole,
but it isin't quite true."
END QUOTE
Invisibly discarding low values, which is what is implied in the other
descriptions, does seem quite different from "flagging" low values. In
the former, the researcher never sees them, or at best forgets they
exist, clearly a bad idea. In the latter, she sees them but decides
not to believe they are reliable. This still would lead her to miss an
important observation in this case, but is (IMHO) rather more
defensible.
Regardless of which version is closer to the truth, it still makes a
very nice story to illustrate the dangers of dealing with "outliers".
It should be pretty obvious that we don't want a computer program to
remove data values automatically without alerting us to what it has
done. The issue of human interpretation of data, and deciding what
points are and are not reliable, is rather more subtle.
Cheers,
Scott
Cereon Genomics
Scott.Chasalow@cereon.com
} -----Original Message-----
} From: STEVEN.P.MILLARD [mailto:STEVEN.P.MILLARD@prodigy.net]
} Sent: Thursday, 16 September, 1999 12:47
} To: S-news (E-mail)
} Cc: 'Keith Dixon'; 'Gene Felber'; 'Yonnie'; 'Edward Malthouse';
} 'Christmann, Andreas'; 'Elaine Florio'
} Subject: [S] References for late discovery of hole in the ozone layer
} due to "outlier" deletion
}
}
}
} Hello Everyone,
}
} Thanks so much for your quick replies to my query about a reference }
citing the delayed discovery of the hole in the ozone layer over }
Antarctica due to software that automatically deleted } "outliers".
Anne } York's response reminded me that NOVA had a program on the
ozone hole } quite a few years ago, and now I'm thinking that is where
I } first heard } about the "outlier" deletion episode.
}
}
} Thanks to Gene D. Felber for pointing out the URL }
http://curriculum.qed.qld.gov.au/kla/eda/ozone.htm which } gives a
brief } overview of this exact topic!!
}
}
} Also thanks to Keith Dixon, who wrote:
}
}
} I believe that the reference is...
} Stolarski, R.S., A.J. Krueger, M.R. Schoeberl, R.D. McPeters, P.A. }
Newman, and J.C. Alpert. 1986. Nimbus 7 satellite measurements of the }
springtime Antarctic ozone decrease. Nature 322:808-811. } On his web
page on "DATA QUALITY CONTROL / QUALITY ASSURANCE" Don } Edwards of the
Department of Statistics, University of South Carolina } describes it
as "One of the most famous data QA/QC blunders occurred } when NASA
computers were programmed to delete satellite } observations of } ozone
concentrations below a specified level, and thus failed to } discover
the "ozone hole" over the south pole (Stolarski et } al. 1986)."
} http://www.lternet.edu/ecoinformatics/guide/edwards.fv4.htm }
NASA's fact sheets usually omit any direct reference to the outlier }
problem. On one hand, they'll say "Since the launch of the } first
TOMS } aboard the Nimbus-7 polar-orbiting satellite in 1978, NASA } has
provided } scientists with reliable, high-resolution daily maps of
global ozone } levels." Yet they also note, "The ozone hole was
discovered through } British ground-based observations in the
mid-1980s, but analysis of } TOMS data indicates it has existed since
at least 1979." It } is left for } the reader to deduce why the
"reliable, high-resolution daily maps" } that NASA was getting since
1978 didn't discover the ozone hole } earlier.
}
} http://pao.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/eart
hsci/ozonestu.htm
Hope that helps,
----------------------------------------------------------------------
Keith W. Dixon _______ _______ ______ _
E-mail: kd@gfdl.gov (_______|_______|______)(_)
URL: http://www.gfdl.gov/~kd/ Geophysical Fluid Dynamics Lab
Postal Address: NOAA/GFDL, | | (_ | ___) | | | | |
P.O. Box 308, Princeton, NJ 08542 | |___) | | | |__/ /| |_____
------------------------------------ \_____/|_| |_____/ |_______)
Sincerely,
--Steve M.
_____________
| *** | Steven P. Millard, Ph.D.
| * |
| * * * | P robability, TEL: 206-528-4877
| * * * | S tatistics & FAX: 206-528-4802
| * | I nformation E-mail: SMillard@ProbStatInfo.com
| * | Web: www.ProbStatInfo.com
| *** | 7723 44th Avenue NE
|___________| Seattle, WA 98115-5117 USA
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