European and Mediterranean Plant Protection Organization
PP 1/152(4)
Organisation Europe
´enne et Me
´diterrane
´enne pour la Protection des Plantes
Efficacy evaluation of plant protection products
Evaluation biologique des produits phytosanitaires
Design and analysis of efficacy evaluation trials
Specific scope
This standard is intended for use in association with EPPO
Standards of set PP 1
Standards for the efficacy evaluation
of plant protection products
and provides detailed advice
on the design and analysis of efficacy evaluation trials.
Specific approval and amendment
First approved in 1989
–
09.
First revision approved in 1998
–
09.
Second revision approved in 2006
–
09.
Revision mainly to reflect zonal assessment approved in
2012
–
09.
Introduction
This standard is intended to provide general background
information on the design and analysis of efficacy evalua-
tion trials. The EPPO Standards for the efficacy evaluation
of plant protection products provide more detailed instruc-
tions on such trials for individual host/pest combinations.
The
set-up
of
a
trial
is
first
considered
(experimental
design, plot size and layout, role and location of untreated
controls). The nature of observations to be made is then
reviewed
(types
of
variables,
modes
of
observation).
Finally, suggestions are made on the statistical analysis of
the results of a trial and of a trial series (estimates of
effects, choice of the statistical test, transformation of vari-
ables). Appendix 1 gives examples of scales used in the
EPPO standards.
What follows is intended to give an outline of good sta-
tistical practice in the analysis of data. It is not, and can-
not be, a prescription for all analyses, and cannot cover
all situations. Practitioners should never underestimate the
need for professional statistical advice. It is important for
practitioners to understand the advice they receive. It is
often better for them to perform a simple analysis that
they can report and defend with confidence, than to accept
advice that leads to an analysis that they may understand
only partially. The bibliography at the end of these stan-
dards may
be
helpful.
It gives
several
good
texts that
attempt to reveal the principles of good statistical practice,
rather than to provide a series of statistical recipes to be
followed blindly.
1. Experimental design
1.1 Experimental scope and objectives
Before the design of a trial is considered, its scope and
objectives should be defined clearly, because these con-
strain the available choices of design. In practice, an itera-
tive
process
is
often
used:
scope
and
objectives
are
gradually adjusted to fit the experimental resources avail-
able. It is vital that the scope and objectives are updated to
reflect decisions made during this process.
The scope of the trial reflects the range of practical out-
comes that may result from the trial and which are rele-
vant to its objectives. Part of the scope relates to the
population which the trial is sampling. Another part deter-
mines the range of environmental conditions, crops, treat-
ment
chemicals,
application
methods
and
target
pests
which the trial is intended to test. The scope defines the
context in which the experimental units and observations
are studied.
The objectives of the trial should be in the form of ques-
tions about the treatments to which answers are desired.
Typical answers will be ‘yes’ or ‘no’, a ranking of treat-
ments or an estimate of a value.
The scope and objectives should form part of the trial
protocol, as described in EPPO Standard PP 1/181
Conduct
and reporting of efficacy evaluation trials, including good
experimental practice
. The planned experimental methods,
design and analysis described below should also form part
of the protocol.
ª
2012 OEPP/EPPO,
Bulletin OEPP/EPPO Bulletin
42
, 367–381
367
Bulletin OEPP/EPPO Bulletin
(2012)
42
(3), 367–381
ISSN 0250-8052. DOI: 10.1111/epp.2610