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Re: Bird Counts and Statistical Validity
- To: Nathan Dias <diasn@yahoo.com>,Charlotte Goedsche <goedsche@mindspring.com>,carolinabirds <carolinabirds@duke.edu>
- Subject: Re: Bird Counts and Statistical Validity
- From: Helmut Mueller <hmuelle@earthlink.net>
- Date: Tue, 18 Feb 2003 16:41:59 -0500
- Cc: <amw25@cornell.edu>
- Delivered-to: pardo@metalab.unc.edu
- In-reply-to: <20030218204131.29368.qmail@web11603.mail.yahoo.com>
- Sender: carolinabirds-owner@acpub.duke.edu
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I have known for some time that birds are not black & white balls in an urn
& that attempts to count them, even when done by "experts" are fraught with
inaccuracy. Maybe we should give up counting & statements about whether
birds are increasing or decreasing?
I am reminded: there are 3 kinds of lies: lies, damned lies & statistics.
Do we need to tell participants in these pseudo-scientific endeavors that
there efforts are worthless?
-- Helmut C. Mueller Ph.D
Professor Emeritus
Dept. of Biology & Curriculum in Ecology
University of North Carolina
Coker Hall CB# 3280
Chapel Hill, NC 27599-3280
Phone: 919-942-4937
Permanent email address: hmueller@email.unc.edu
> From: Nathan Dias <diasn@yahoo.com>
> Date: Tue, 18 Feb 2003 12:41:31 -0800 (PST)
> To: Charlotte Goedsche <goedsche@mindspring.com>, carolinabirds
> <carolinabirds@duke.edu>
> Cc: amw25@cornell.edu
> Subject: Bird Counts and Statistical Validity
>
> Charlotte -
>
> Just because so-called "scientific" studies use
> Christmas count results does not mean they would
> withstand rigorous evaluation by a statistician.
>
> Lookit:
> When considering studies: in general, samples are
> taken and inferences about the overall population are
> made by comparing samples. If one cannot trust the
> samples themselves, one cannot place much trust in the
> inferences drawn from their comparison. The name of
> the game in applied statistics is to minimize the
> uncertainty attached to these inferences. I shall
> demonstrate how this cannot be done to an acceptable
> degree with data resulting from Christmas Counts and
> GBBC. My claim was not irresponsible, Charlotte; it
> is founded in statistical bedrock.
>
> Here are some barriers to studies based on GBBC and
> Christmas Count results being statistically valid:
>
> - Too many novice/untrained/kooky observers (who might
> misidentify birds). Both the spatial distribution and
> the number of observers of varying competence is
> uneven and non-random from one count to the next.
> STRIKE ONE. Example: A phone survey/poll asks
> respondents questions with cut-and-dry answers. The
> folks compiling the results "know what they are up
> against" (they know for sure that the respondent said
> they favored the U.S. going to war with Iraq). But
> folks using Xmas counts or GBBC results don't know for
> sure if Suzy Dimwit really saw a given Loggerhead
> Shrike or misidentified a Mockingbird. This is a
> hindrance to having a representative sample.
>
> - Grossly inaccurate numeric data (see my previous
> post on double-counting, under-counting,
> mis-estimation of flocks, etc.). Everyone knows that
> Christmas count species totals that folks turn in are
> ballpark guesstimations (way too high or too low) in
> many cases. This numeric data is also inconsistent
> (and non-random) in its inaccuracy. STRIKE TWO. In
> and of itself, this doesn't mean a study based on the
> data is not statistically valid, but it does mean the
> uncertainty attached to inferences drawn is so high as
> to make the inferences of much less use.
>
> - Uneven/inconsistent coverage of the same physical
> area(s) from year to year. STRIKE THREE. This can
> make comparing data from different years like
> comparing apples to oranges. NONREPRESENTATIVE
> SAMPLES result from this practice (a cardinal sin if
> one is seeking statistical validity). Example: many
> christmas counts have sub-areas that are not
> "glamorous." They receive little or no coverage some
> years, half-hearted coverage in others and good
> coverage by heroic experts in others. This coverage
> is uneven and nonrandom. The same argument applies to
> zip codes in the GBBC. Another uneven coverage
> problem results from the need for boats to cover water
> areas of Christmas Counts. Another uneven coverage
> problem (on Christmas Counts) happens when land
> (especially plantations) gets sold to
> non-birder-friendly landowners. Having coverage
> depend on the whims of a cranky landowner is
> definitely inconsistent and nonrandom.
>
> - Due to the relatively low number of observers versus
> total bird population size, the high number of
> non-expert observers, inconsistent sample area
> coverage, inconsistently inaccurate data and many
> other factors: Xmas Counts and GBBC produce
> unacceptably high probabilities of alpha error and
> beta error. STRIKE FOUR. -- Alpha error is when one
> sees a statistically significant difference in the
> samples, when no difference actually exists in the
> populations. Beta error is when one does not see a
> statistically significant difference in the samples,
> when a difference actually does exist in the
> populations. When probabilities of alpha error and
> beta error are too high, one can throw inferences
> drawn out the window.
>
> I could go on and on with the strikes. The
> nonrepresentative sample issue alone means NOT
> STATISTICAL VALID and it keeps cropping up due to
> various considerations. QED.
>
> Breeding Bird Atlas studies are much better (in terms
> of statistical validity) than Xmas counts and GBBC -
> they use highly competent observers, identical routes
> (areas of coverage) over time, identical time spend
> traversing the route from year to year, and more.
>
> Nathan Dias - Charleston, SC.
>
> PS My father (who has a PhD in Biostatistics) also
> gets red in the face at the thought of folks claiming
> that studies based on Christmas Count and GBBC results
> are statistically valid. As someone with degrees in
> (pure) math and Computer Science, as well as A grades
> in graduate level courses in Statistics, I agree
> whole-heartedly.
>
> --- Charlotte Goedsche <goedsche@mindspring.com>
> wrote:
>> As a person who birds for fun AND is interested in
>> bird distribution, I'd
>> like to address two issues Nathan Dias brought up in
>> a recent posting on the
>> Great Backyard Bird Count and the Christmas Bird
>> Count, both conducted in
>> North America.
>>
>> His claim that Christmas Bird Counts do not produce
>> "hard scientific
>> (statistically valid) data" is simply irresponsible.
>> Check out the following
>> website for a vast bibliography of scientific
>> studies based on Christmas
>> Bird Counts: http://www.audubon.org/bird/cbc/bb.html
>> . (Since the GBBC is
>> only in its 6th year, it is, of course, too early to
>> expect published
>> scientific results from it.)
>>
>> Checklist programs similar to the GBBC have been
>> used in Canada for many
>> years, by the way. See
>> http://www.oiseauxqc.org/feuillets/cbcp_i_e.html for
>> a history of the Canadian checklist program and some
>> of the publications
>> based on it.
>>
>> While Nathan's claim that "Gut feelings by
>> experienced birders are of
>> about as much use [as the GBBC] (and more likely to
>> spot trends
>> quicker)" may have some validity on the local level,
>> it ignores the
>> geographic and temporal scope of the GBBC, as well
>> as the organizational
>> framework it provides. The GBBC (like other counts)
>> collects masses of
>> personal observations and transforms them into data
>> which can be shared and
>> compared, and will eventually provide valid
>> information on late-winter
>> population dynamics and trends which I expect will
>> lead to scientific
>> publications.
>>
>> So have fun birding, and share the observations you
>> made between 14 and 17
>> February by submitting them to the Great Backyard
>> Bird Count,
>> http://www.birdsource.org/gbbc/ . Checklists will be
>> accepted through 28
>> February, so it's not too late to send in your
>> sightings. And don't forget
>> that at the same site you can also see what species
>> have been reported
>> where, and view maps to compare the distribution of
>> species of interest to
>> you over the years.
>>
>> Good birding!
>>
>> Charlotte
>> Charlotte Goedsche
>> Weaverville NC
>> goedsche@mindspring.com
>>
>
>
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