1.2 Types of design
EPPO Standards for the efficacy evaluation of plant protec-
tion products envisage
trials in which the
experimental
treatments are the ‘
test product(s), reference product(s) and
untreated control, arranged in a suitable statistical design
’.
It is also envisaged that the products may be tested at diff-
erent doses and/or application times. This applies particu-
larly to the use of a higher dose in selectivity trials and
dose-response studies in general.
Mono-factorial designs are appropriate for trials if the test
product(s), reference product(s) and untreated control can be
considered as different levels of a single factor, and if there
are no other factors that require study. However, if, for
example, the effect of each product in an efficacy trial is to
be studied at different doses, then a factorial design may be
used with, in general, all possible combinations of treatments
from both factors represented. In this way, important interac-
tions between the factors may be revealed and estimated.
The principal randomized designs which are likely to be
used are: completely randomized and randomized complete
block. These are illustrated below on the basis of a mono-
factorial example with 8 treatments, i.e. 5 different test
products, 2 reference products and an untreated control;
each treatment is replicated 4 times.
1.2.1 Completely randomized design
The treatments in a completely randomized design (Fig. 1)
are
assigned
at
random
to
the
experimental
unit.
This
design is potentially the most powerful statistically (in the
sense that there is a maximum chance of detecting a signifi-
cant difference if it exists), because it allows retention of
the maximum number of degrees of freedom for the resid-
ual variance. However, it is suitable only if the trial area is
known to offer a homogeneous environment. If there is
considerable heterogeneity between different parts of the
trial area, residual variance may become unacceptably high,
and it is better to use a design that accounts for this, such
as a randomized complete block.
1.2.2 Randomized complete block design
A block is a group of plots within which the environment
relevant to the observations to be made is homogeneous. In
this design, the blocks are laid out deliberately so that plots
within them are as uniform as possible before application
of treatments. Usually, each treatment appears once and
once only, within each block. The treatments are distributed
randomly to the plots within the blocks, which act as repli-
cates. The arrangement of treatments in each block should
be randomized separately for each block. In the following
examples (Figs 2
–
4), there are 4 blocks and 8 treatments.
The layout of the blocks aims to control the heterogeneity
of the site (e.g. slope, direction of work at sowing or plant-
ing, exposure, degree of infestation etc.), plants (size, age,
vigour) or of the conditions occurring during the experi-
ment (application of treatments, assessments). The layout of
the blocks therefore requires some preliminary knowledge
of the trial area. The arrangement of plots within blocks
may be influenced by plot shape: long narrow plots are
often arranged side-by-side, whereas, square plots may be
laid out in other ways. However, blocks do not have to be
placed side by side. If there is good preliminary knowledge
of a field, this may be utilized by scattering blocks across
the field, to account for previously observed heterogeneity
(Figs 5 and 6). Although it is quite possible that in a ran-
domized layout, treatments within a replicate may appear in
treatment order, this is to be avoided wherever possible in
the interests of unbiased evaluations. If there is extremely
good
preliminary
knowledge,
and
it
can
be
confidently
assumed that conditions will remain the same for the exper-
iment to be done, complex heterogeneity may be allowed
for, and it is not even necessary for plots of the same block
to be adjacent. For example, blocks may be broken up to
account for a known patchy infestation of nematodes. In
Fig. 6, plots within block 1 have been deliberately placed
at points of visibly low infestation and plots within block 2
at points of visibly high infestation.
2
7
3
7
8
3
5
4
1
2
6
2
2
3
4
6
8
4
5
4
6
8
1
5
1
5
7
8
1
7
3
6
Fig. 1
A fully randomized design. Each treatment (labelled 1
–
8) is
replicated 4 times; individual treatment labels are assigned completely
randomly to the 32 plots.
Block 1
3
8
7
2
5
4
6
1
Block 2
4
7
5
1
6
2
8
3
Block 3
5
6
7
2
8
3
1
4
Block 4
8
4
1
3
5
6
7
2
Block 1
Block 2
Block 3
Block 4
5
7
1
2
8
4
3
6
4
6
1
5
3
8
2
7
3
8
2
5
4
7
6
1
2
3
1
8
5
6
7
4
Gradient
Fig. 2
Possible arrangement of blocks and plots in randomized blocks in field trials. An environmental gradient down the field is accounted
for, either by arranging blocks down the gradient, or by placing blocks side by side. In each case, plots within blocks placed across the gradient are
affected equally by the environmental variable.
368
Efficacy evaluation of plant protection products
ª
2012 OEPP/EPPO,
Bulletin OEPP/EPPO Bulletin
42
, 367–381