[R] SE for combined data

AJ Smit ajsmit at science.uct.ac.za
Mon Apr 19 10:15:56 CEST 2004

Dear all

I have just had the question from a colleague. I know that it is not directly related to R (I will probably use R to do the analysis), but I hope someone can give us some insight:

AJ Smit

I sampled populations of a seaweed in the intertidal in order to estimate
the standing biomass of that seaweed at that site.

Due to clumped distribution patterns, I chose a stratified sampling system,
as follows. In each of three subjectively defined biomass classes (low,
medium and high biomass density), four quadrats (usually) were haphazardly
placed, and the biomass in those quadrats harvested. This provided an
estimate of the biomass density present in that biomass density class. The
area of ground covered by that biomass density class was also estimated,
and, by combining the estimated biomass density and the area covered by that
biomass density class, the total biomass in that biomass density class was
estimated. When the estimated biomass in the three biomass density classes
was combined, I had a figure for the standing biomass for that site..

So, for each biomass density class, I have a number of biomass density
estimates (usually, but not always, four), and an estimate of the area
covered by that class.

I repeated this at a number of sites. Biomass density classes were not
necessarily the same between sites.

Given that I can calculate measures of variation for each biomass density
class, is there a way to combine these data, presumably weighted by the area
covered by each biomass density class, and calculate the standard error for
the final biomass estimate at each site?

Dr Albertus J. Smit
Department of Botany
University of Cape Town
Private Bag Rondebosch
Cape Town
Tel. +27 21 689 3032
Fax  +27 21 650 4041

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