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Re: Fwd: RE: Blade article
- Subject: Re: Fwd: RE: Blade article
- From: "Richard L Whitman" <rwhitman@usgs.gov>
- Date: Wed, 1 Sep 2004 09:52:27 -0500
- Cc: beachnet@great-lakes.net
- Delivered-To: beachnet-archive@glc.org
- Delivered-To: beachnet@great-lakes.net
I am not the author of the article referred to paper, but I can explain why
one uses geometric means.
A simple arithmetic means, also called an average, is an estimate of the
population and assumes that the samples are representative of the true
population. Since this almost never completely true, we invented
statistics to help us estimate of the characteristics of the population
(range, variation, distribution, mean, trends).
E. coli, like most biological populations, are not normally distributed,
they are most often clumped (clustered, contagious, patchy, etc). In
statistical terms, this means the variance exceeds the mean. There are
several solutions to this. One way is to take the median. This is the
sort of thing they do with people's incomes or home prices. That is
because while most of us are average, those mega-rich really skew the mean
upwards. There are two more ways that we use to make the sampled
population more normal.
Composite sampling from many places on the beach will give you a better
representation than single samples. This is not without cost though, you
lose information on the variation which is important for most statistical
testing. Here is a seemingly unlikely example, but I've seen this sortof
data often. You took 5 samples each reading 50, 50, 100, 100, 1000. The
compositing of those samples would yield 260 cfu/100ml, a beach closure.
If you had taken the samples individually you would have found that the
standard deviation was 414 and you would know that there was a problem with
your estimate of the average. We look at sampling strategy closely in the
August issue of Env. Sci and Tech. Julie Kinzelman and Al Dufour have
worked with composite samplings a lot and have some actual data on this.
The second way to deal with the extremes, is to log transform the data.
Microbiologist traditionally use 10 based logs. This has the effect of
elevating low numbers and reducing high numbers. In effect, this allows
for a more bell shape distribution, a population characteristic that is
necessary for most traditional statistical testing.
The geometric mean can be computed by:
1. taking the logarithm of each
number
2. computing the arithmetic mean of
the logarithms
3. raising the base used to take the
logarithms to the arithmetic mean
Here is an example
X
Log(X)
1
0.0000
2
0.30103
3
0.47712
10
1.00000
Geometric mean = 2.78
Arithmetic mean = 0.44454.
10.44454 = 2.78
If any one of the scores is zero then
the geometric mean doesn't make any
sense and cheat by adding a constant to
every number. The geometric mean for
the example I gave intially is 120,
swimmable.
The geometric mean is always lower than
the arithmetic mean, so the criteria is
different. EPA can explain how they
derived the 126 CFU/100 ml. Hopes this
helps a bit.
Richard Whitman
Chief, Lake Michigan Ecological Research Station
219-926-8336 Ext. 424
1100 North Mineral Springs Road
Porter, IN 46304
"Shannon Briggs"
<BRIGGSSL@michiga To: <richard_whitman@usgs.gov>
n.gov> cc:
Subject: Fwd: RE: Blade article
09/01/2004 06:50
AM
Good Morning Richard,
Are you following these emails? Are they talking about one of the
projects that you recently published? You may want to direct readers to
the published article or send a note thru beachnet to lead us out of the
dark. Or better yet, write a brief explanation thru beachnet list serv
and tell them that you will be happy to provide a thorough explanation
and provide all sorts of wisdom at the Great Lakes Beach Conference!
Your work and insight are helpful to so many of us! We need to learn
from you!
Shannon
>>> "Taylor, Christopher" <Christopher.Taylor@ci.toledo.oh.us> 08/31/04
03:45PM >>>
Got me... Maybe the author is referring to the fact that the geometric
mean
will smooth out either hi or low outliers. The geometric mean has
tended to
confuse many people, and many times when I have tried to explain it to
someone, unless they have a math/science background, I usually get a
blank
stare. The author is more aware than many of what the geometric mean
is, but
taking up inches of column to try to explain it usually doesn't make it
past
the editors...
Chris Taylor
Chief Chemist
Toledo Water Plant
419-245-1717
-----Original Message-----
From: Culp, Jason [mailto:jculp@stcatharines.ca]
Sent: Tuesday, August 31, 2004 11:37 AM
To: beachnet@great-lakes.net
Subject: RE: Blade article
With regards to this article could someone please clarify what the
other
various factors are, and how they are weighted in the formula.
"The state health department makes its recommendations for postings
based
on a complicated logarithmic formula in which the five latest test
results are converted into what's known as a geometric mean, with the
average weighted by various factors."
Thanks
Jason Culp
City of St. Catharines - Environmental Services
Lake Street Service Centre
383 Lake Street L2N 4H5
phone: 905-688-5601 ext.2196
e-mail: jculp@stcatharines.ca <mailto:jculp@stcatharines.ca>
Bacterial levels lower at area beaches (Toledo Blade)
By TOM HENRY
August 1, 2004
With the Dog Days of August upon us today, people generally have about
a
month left to enjoy themselves at area beaches before settling into
fall
activities after the Labor Day weekend.
This August, they can take some comfort knowing that the bacteria
levels
at northwest Ohio's public beaches have - for the most part - been
slightly lower than normal.
With the exception of Lakeview beach in Lorain County and Catawba
Island
State Park in Ottawa County, the situation has been a little better
than
in past years. Swimming advisories have been in effect at Lakeview all
but one day since June 8, while advisory signs went up at Catawba
Island
State Park on July 20.
Swimming advisories also were posted Wednesday for the first time this
summer at Maumee Bay State Park's inland pond. The park's Lake Erie
beach had advisories posted for two brief interludes in June, but made
it through July without warnings issued - one of the few times that
has
happened over the past decade.
Overall, northwest Ohio has had fewer postings this year than in
recent
years. Port Clinton's beach, plagued by bacteria much of last summer,
hasn't had a single posting this summer.
Mike Oricko, Lucas County environmental health director, said he and
other veteran bacteria-trackers are hardly patting themselves on the
back and getting complacent. They acknowledge the low counts are
largely
a reflection of this summer's relatively calm weather.
The general advice: Don't swim within 24 hours of a storm or if strong
winds cause a lot of wave action. Both stir up sediment in which
bacteria can hide, Mr. Oricko said.
The always-fickle situation at Maumee Bay State Park has officials on
their toes again. The park has recorded scattered, single-day bacteria
spikes the past two weeks, as high as 308 colonies per 100 milliliters
of E. coli bacteria at its Lake Erie beach on Monday and 365 colonies
per 100 milliliters at its inland beach on Tuesday, records show.
But the highs and lows have been somewhat erratic, rejuvenating the
debate over whether the Ohio Department of Health's trending analysis
sufficiently protects public health. The advisories are merely a
warning
and public access to the affected waters is not barred.
The state health department makes its recommendations for postings
based
on a complicated logarithmic formula in which the five latest test
results are converted into what's known as a geometric mean, with the
average weighted by various factors.
When the mean level exceeds 126 colonies per 100 milliliters of E.
coli
bacteria, the agency recommends that the signs go up. Critics have
argued the time lag is too long, because it takes several days of
sustained high readings to drive up the average.
"Nobody likes it. But it's the best we have now," Mr. Oricko said. "We
need something immediate."
The U.S. Environmental Protection Agency is in the midst of a
multiple-year study to develop a faster technique for determining
beach
bacteria - in as little as two hours - thereby providing swimmers the
convenience of same-day results. The current protocol at Ohio's public
beaches requires a turnaround time of more than 24 hours in the
laboratory, then results get sent off to the state Department of
Health's headquarters in Columbus for another day of review.
The potential for a new evaluation technique is being researched by a
U.S. EPA laboratory in Chapel Hill, N.C. The study is expected to take
at least four years, with the first two years spent in the field at
four
freshwater Great Lakes beaches.
Huntington Beach in Cleveland was one of two studied last year. The
other was West Beach in Indiana, part of the Indiana Dunes National
Lakeshore. Silver Beach in St. Joseph, Mich., and Washington Park
beach
in Michigan City, Ind., were used for this summer's pilot studies,
said
Dick David, a U.S. EPA spokesman.
Timothy J. Wade, a U.S. EPA epidemiologist involved with the study,
said
the research will move on to saltwater beaches the next two years.
While results from Huntington and other Great Lakes beaches are too
preliminary to discuss in detail, they "seem promising, but there are
caveats to that statement," Mr. Wade said. He said he plans to present
an outline of the project Tuesday at the International Society for
Environmental Epidemiology in New York.
The potential new evaluation technique would shorten the time lag by
having lab scientists focus on the DNA of a specific strain of
bacteria
other than E. coli. The longstanding protocol requires waterborne
bacteria samples to be cultured and grown overnight for study.
The U.S. EPA interviewed more than 5,000 swimmers last year and hopes
to
interview at least another 5,000 this summer and in future years.
Participants receive follow-up phone calls 10 to 12 days after they
have
visited beaches to determine if they contracted any health problems.
U.S. EPA scientists then evaluate whether any DNA symptoms they might
have found during their two-hour lab research can be relied upon to
predict whatever health problems swimmers may have encountered, such
as
nausea, cramps, or vomiting, Mr. Wade said.
Numerous Great Lakes beaches have been plagued for years by high
bacteria counts, leading people to wonder if pollution has gotten
worse
or if the scientific ability to detect a problem has just become more
sophisticated.
Maumee Bay State Park's Lake Erie beach has traditionally been one of
the problem areas. Yet 30 miles to the east, one of the state's oldest
public beaches at East Harbor State Park has consistently ranked among
the cleanest.
Theories about the bacteria problem at Maumee Bay State Park's Lake
Erie
beach have ranged from animal runoff in nearby ditches to human waste
from faulty septic tanks. Many of the septic problems have been
resolved, officials have said. They have broadened their search at
times
to include sewage discharges from the Detroit River, and the interplay
between Maumee River runoff and the powerful Lake Erie water intake of
FirstEnergy Corp.'s coal-fired Bay Shore power plant.
But none of those theories has provided a smoking gun. The search
continues for the primary bacteria sources.
The problem at Maumee Bay's man-made inland beach has been easier to
control, because it has been attributed largely to excessive droppings
from ducks and geese. Officials are keeping their fingers crossed for
a
mild August.
"The weather has been cooler. We haven't had the major storm events
driving the waves into the shorelines and up from ditches," Jim
Brower,
park manager, said.
Mr. Oricko agreed: "To be perfectly honest, the weather deserves some
credit here."
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----- Forwarded by JohnG Lyon/LV/USEPA/US on 08/09/2004 07:01 AM -----
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