[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Bird Counts and Statistical Validity



You guys are ruining all the enjoyment of counting the birds.  The fact that
it gets new people interested in birds at all is reason enough to have it.
I'm sure the people at Cornell are aware of all the things you both say, but
if all the surveys sent in from an area report large numbers of goldfinches,
for instance, that does say something.  There's no survey in the world that
can give actual numbers to something as mobile as a bird--so why not just
enjoy it.  It's not meant to be perfect, it's meant to be interesting and
fun and a learning tool.  Me thinketh thou protesteth too much.
Bruce Krucke
Yonges Island, SC
www.toogoodoostudios.com


----- Original Message -----
From: Nathan Dias
To: Charlotte Goedsche ; carolinabirds
Cc: amw25@cornell.edu
Sent: Tuesday, February 18, 2003 3:41 PM
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
>


__________________________________________________
Do you Yahoo!?
Yahoo! Shopping - Send Flowers for Valentine's Day
http://shopping.yahoo.com