[R-sig-Geo] rGeo vs. gstat; Question on geostatistical method for specific experimental design

[Edzer J. Pebesma]

Schlatter Christian wrote:

>Dear Edzer, dear list members
>
>Thank you very much for your comments. It made me investigate quite well. 
>
>The answer to the question "rGeo vs. gstat" is answered by your very helpful article of the DSC 2003 meeting in Vienna (chapter "introduction"): http://www.ci.tuwien.ac.at/Conferences/DSC-2003/Proceedings/Pebesma.pdf
>
>As I'm new and not yet so familiar with the customs of [R-sig-Geo], I did not feel about asking about my personal statistical problems but more about general statistical questions. 
>
>But as you asked for more detail I will gladly describe them (please let me know if this should not be the place for it):
>
>We are looking at parasitism rates in cabbage pests eggs (Lepidoptera: mainly Mamestra brassicae, Pieris rapae) in relation to distance effects from flowering strips. Specifically the following question: In what relation is the parasitism rate to the distance from the flower source (which in the case are sown flower strips). 
>
>Many practical restrictions led to a somehow "compromisical" experimental design consisting of four blocks in one cabbage field (two with flower strips, two without). Each block has two grids of 6x 8 plants (distance in between each plant: 3m) on each side of the flower strip (cp. Image), 96 plants per block. On each plant we collected pest eggs to determine parasitism rate. (Addtionaly we sampled on 4 different days).
>
>The main problem is the low parasitism rate in the field (only about 15-20% of all eggs have been parasitized), consequently many 0 values. 
>
>My idea was to calculate for each of the four blocks variogram parameters and to compare them afterwards (as we had two study sites, we have 8 blocks all in all, making 4 with flowers and 4 without). With so many "0" samples difficult to manage. 
>
>Now I found in Edzers article and the gstat manual the possibility of block kriging. Without knowing exactly what it is, I suppose there is the possibility to keep all four blocks together and defining the blocks with the rectangular block-defining possibility. 
>
>Is this a valid procedure?
>  
>
Before you apply block kriging I would strongly encourage you to
read an appropriate text book about what block kriging is. Blocks
in geostatistics usually refer to specific spatially contiguous regions, not
a combined effect of blocking in experimental design. Kriging refers
to spatial prediction, not the estimation of effect size.

Best regards,
--
Edzer