A Field Guide to Experimental 
Designs

Experiment with pseudo-replication

To perform an experiment a tad simpler, it might seem best to the researcher to just apply a treatment to all the trees in a row, and then the next treatment to all the trees in the next row, etc. The experiment is being replicated isn't it by the number of trees in each row?

Unfortunately, this experiment cannot be legitmately analysed because the null hypothesis cannot be tested. If treatments are applied to whole rows, then treatment should be applied to replicate rows. One such design, the split-block, is used in such circumstances.

Field marks:

  • Treatments are applied to whole groups of experimental subject at a time.
  • There is no replication across the whole set of experimental subjects.

Sample layout:
Different colors represent different treatments. There are four treatments (A-D) applied to four rows (I-IV).

Split-split plot sample layout

Row I     A  A  A  A
Row II    B  B  B  B
Row III   C  C  C  C
Row IV    D  D  D  D

 

ANOVA table format:

Source of
variation
Degrees of
freedom
Sums of
squares (SSQ)
Mean
square (MS)
F
None

Compare with:

  • Randomized Complete Block (RCB): treatments are assigned at random within blocks of adjacent subjects, each treatment once per block.
  • Split-block design: treatments are applied in strips across the entire block: one set of treatments is randomized in one direction; a second set is randomized in a second direction.

 

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Friday, August 25, 2000