...
http://www.ices.dk/reports/FTC/2005/WKSAD05.pdf
...
Random
...
vs
...
Systematic
...
Sampling:
...
- Systematic
...
- Sampling:
...
- they
...
- used
...
- a
...
- grid
...
- of
...
- 64
...
- equally
...
- spaced
...
- points,
...
- the
...
- location
...
- of
...
- initial
...
- point
...
- was
...
- randomly
...
- selected.
...
- basically,
...
- they
...
- divided
...
- the
...
- survey
...
- area
...
- into
...
- 64
...
- equal
...
- boxes.
...
- They
...
- then
...
- randomly
...
- chose
...
- a
...
- point
...
- in
...
- one
...
- box,
...
- and
...
- then
...
- collected
...
- the
...
- sample
...
- at
...
- the
...
- same
...
- relative
...
- location
...
- in
...
- each
...
- of
...
- the
...
- 64
...
- boxes.
...
- I
...
- think
...
- that
...
- this
...
- was
...
- continued
...
- (i.e.
...
- they
...
- generated
...
- another
...
- regular
...
- grid
...
- of
...
- 64
...
- points)
...
- until
...
- they
...
- had
...
- 1000
...
- points
...
- in
...
- total.
...
- Random:
...
- start
...
- with
...
- 64
...
- stations,
...
- then
...
- add
...
- random
...
- stations
...
- and
...
- use
...
- an
...
- algorithm
...
- to
...
- see
...
- how
...
- long
...
- it
...
- takes
...
- to
...
- travel
...
- from
...
- each
...
- station
...
- to
...
- the
...
- next
...
- one.
...
- They
...
- continued
...
- adding
...
- random
...
- stations
...
- until
...
- they
...
- reached
...
- a
...
- maximum
...
- time
...
- limit
...
- (making
...
- the
...
- number
...
- of
...
- sample
...
- locations
...
- variable).
...
They
...
ran
...
these
...
two
...
survey
...
types
...
on
...
two
...
different
...
simulated
...
oceans
...
of
...
fish.
...
One
...
ocean
...
had
...
high
...
variability
...
and
...
low
...
spatial
...
autocorrelation
...
(don't
...
know
...
what
...
that
...
is),
...
and
...
the
...
other
...
had
...
low
...
variability
...
and
...
high
...
correlation.
...
(each
...
ocean
...
had
...
1x10^7
...
fish).
...
The
...
random
...
sample
...
did
...
a
...
better
...
job
...
estimating
...
the
...
population
...
in
...
the
...
high
...
variability
...
ocean,
...
but
...
the
...
systematic
...
sample
...
did
...
a
...
better
...
job
...
with
...
low
...
variability.
...
Implies
...
that
...
sample
...
method
...
selection
...
should
...
correlate
...
to
...
spatial
...
distribution
...
of
...
fish.
...
("Further
...
investigation
...
of
...
a
...
wider
...
range
...
of
...
surfaces
...
with
...
different
...
properties
...
should
...
help
...
to
...
refine
...
the
...
parameters
...
that
...
influence
...
the
...
point
...
at
...
which
...
different
...
survey
...
strategies
...
are
...
more
...
efficient
...
estimators...")
...
Stratification:
...
The
...
international
...
bottom
...
trawl
...
survey
...
used
...
stratification
...
based
...
on
...
depth.
...
There
...
is
...
evidence
...
that
...
in
...
some
...
areas,
...
bottom
...
sediment
...
type
...
makes
...
a
...
significant
...
difference
...
in
...
fish
...
populations,
...
so
...
characteristics
...
of
...
the
...
seabed
...
will
...
be
...
incorporated
...
into
...
future
...
stratification
...
designs.
...
Stratification
...
can
...
become
...
a
...
confounding
...
factor
...
in
...
samples
...
of
...
several
...
different
...
species
...
where
...
the
...
species
...
have
...
different
...
spatial
...
distribution
...
characteristics.
...
Preliminary
...
analysis
...
is
...
need
...
to
...
know
...
how
...
to
...
stratify
...
when
...
surveying
...
several
...
species.
...
(this
...
is
...
where
...
I
...
got
...
bored
...
of
...
reading
...
most
...
of
...
the
...
document,
...
and
...
it
...
is
...
174
...
pages,
...
so
...
the
...
rest
...
is
...
just
...
a
...
summary
...
of
...
a
...
skim
...
reading).
...
There
...
is
...
a
...
detailed
...
graphic
...
on
...
printed
...
page
...
24
...
that
...
describes
...
how
...
to
...
chose
...
what
...
type
...
of
...
survey
...
to
...
use
...
to
...
get
...
good
...
population
...
estimates.
...
Starting
...
on
...
printed
...
page
...
39
...
the
...
report
...
talks
...
about
...
integrating
...
trawl
...
survey
...
data
...
and
...
SONAR
...
data.
...
Printed
...
page
...
49
...
starts
...
talking
...
about
...
estimating
...
population
...
parameters
...
from
...
survey
...
data.
...
Recommendation
- The spatial distribution of the fish should be considered when designing and
analysing surveys. A decision tree has been provided to assist in the choice of
methods available. Survey planners should be fully aware of the assumptions allied
to any model-based estimation technique.- The survey specific effect of tow duration, should be investigated in individual
surveys. Shorter tows should be implemented if found to provide an improvement
in the precision of the survey.- Covariates should be used, if available, where they provide an improvement in
the precision of the survey. Be aware that the covariates must have a good relationship
with the response and be available over the entire sample space (not just
the sampled area).- Inverse variance weighting should be considered to combine survey data.
When combining indices of the same resource, the inverse variance of the individual
indices is a useful weighting scheme.- The effective sample size to determine biological parameters should be investigated.
The effective sample size of fish selected for ageing, measuring, etc. can
be much smaller than the actual number of animals sampled, it is, therefore, important
to account for this when reporting information on biological parameters.
In cases where this can demonstrated to be smaller than current sample sizes
more effort can be incorporated into sampling other species (including non-fish
species) for consideration of an ecosystems approach (e.g. to compile community-
based indicators).- Quantiles of individual distributions can be used to map biological data rather
than interpolating a summary statistic (e.g. mean length).