Ergebnis für URL: http://alexei.nfshost.com/PopEcol/lec2/stratif.html2.4. Stratified sampling
Stratified sampling is used if sampled area (or volume) is heterogeneous (Pielou,
p.107). If patch size is much larger than inter-sample distance, then kriging can
be used instead of stratified sampling.
In stratified sampling program, the area (volume) is subdivided into 2 or more
portions which are sampled separately. Example: pine sawflies prefer to spin
their cocoons close to the tree; thus, the area adjacent to trees (within 1 m
radius) can be sampled separately from the rest of the area.
1. Density of samples in each stratum should be proportional to the variance of
organism counts in the stratum -- in this case the maximum precision is reached.
Taylor's power law can be used to predict variance from the mean. Variance
usually increases with mean, and thus, the stratum with higher organism density
should be sampled more intensively.
2. The mean population density, M, is estimated as a weighted mean of mean
densities, M[i], in each stratum, i, with weights, w[i] equal to the area covered
by stratum i:
[eqstrat1.gif]
3. The standard error, SE, of the mean is equal to
[eqstrat2.gif]
where SE[i] is the standard error for the mean in stratum i.
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[4]Alexei Sharov 1/12/96
References
1. http://alexei.nfshost.com/PopEcol/lec2/geostat.html
2. http://alexei.nfshost.com/PopEcol/lec2/sampling.html
3. http://alexei.nfshost.com/PopEcol/lec2/caprecap.html
4. http://alexei.nfshost.com/~sharov/alexei.html
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