HOW TO PERFORM AN
G. STUART PETTYGROVE
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
Revised October 1981
Local technicians in developing countries increasingly are
being called upon to test innovative measures developed by
agricultural researchers at the national or regional level.
Improved plant varieties, new fertilization practices,
pesticides, new feed mixtures, and improved harvest
procedures are just a few of the more important innovations
that must be thoroughly tested at the local level before
are passed on through extension methods to the farmer.
Local research often is not carried out by trained research
personnel, but by extension agents, teachers, training
workers, community development agents, foreign technicians,
fertilizer and seed distributors, and farmers with large
The purpose of this book is to show local farmers and others
the basic steps to design, execute, and measure an
experiment. This book does not cover statistical anlysis; it
assumed that trained statisticians are available for this
TABLE OF CONTENTS
List of Figures
SECTION I. SOME BASIC CONCEPTS
I. The Need for
II. An Experiment
Versus A Demonstration
III. Some Basic
Concepts in Statistics
A. The normal
B. The null
SECTION II. HOW TO PERFORM AN AGRICULTURAL EXPERIMENT
II. Designing the
C. Selection of
D. Selecting the
E. Plot size and
III. Execution of the Experiment
A. How to lay
out a right angle
B. Labeling and
IV. Measuring and
Recording the Results
A. When should
measurements be taken?
B. What should
C. Put all
observations in numerical terms
D. A report
Appendix: Table of Random Numbers
LIST OF FIGURES
1. Normal curve
2. Normal curves
with and without fertilizer
3. The completely
4. Random complete
5. Random complete
block suitable for demonstration
6. How to make
random the Latin square design
8. Plot shape
9-A. Laying out a right angle
9-B. Laying out a right angle
9-C. Laying out a right angle
SOME BASIC CONCEPTS
I. THE NEED FOR LOCAL RESEARCH
Many countries today are experiencing what is called
development." Basically, this means three things for
agriculture: (1) it has become more complex technically; (2)
has become less oriented to home consumption and more
to the market; (3) it has become dynamic; that is, it is not
simply moving to a new, more efficient level of operation,
is in a continuous state of flux.
In many countries, research facilities have been established
the national and regional level. New plant varieties and
cultural practices are being tested successfully at
these facilities. But before they can have any effect on
production, they must be tested thoroughly at research
schools, and farms on the local level.
The basic problem facing local experimenters is whether the
of a new or different practice will affect the outcome of
particular segment of agricultural enterprise in their area.
so, to what extent? If farmers fail to adopt a beneficial
because it has not been tested locally, or if they adopt a
harmful practice because it has been tested improperly,
extension agents and those who have carried out experiments
must share the blame.
Local personnel have a great responsibility to become
in testing and evaluating new practices so that they may
such mistakes. If great care is exercised, untrained
can become sufficiently expert in experimentation to bring
benefits to the local farmers and, hence, to the entire
II. AN EXPERIMENT VERSUS A DEMONSTRATION
What is an experiment?
An experiment is a test or tentative procedure for the
of discovering something unknown, or for testing a principle
supposition. It must be carried out in an unbiased manner.
assumptions are made regarding the outcome; the results must
always be accepted. If we suspect that the results are not
typical, we still must accept them, although we should perform
the experiment again. In an experiment, treatments are
or repeated, and they are arranged in test plots or as
random units in a procedure.
An observation trial is not used to draw any experimental
but may determine if a practice is worth testing.
A result trial on a farm is the testing or demonstration of
single practice that has been proven elsewhere, but which is
still unproven in the farmer's mind.
What is a demonstration?
A demonstration shows a response that already has been
in an experiment. It is not conducted according to the
for an experiment, and therefore cannot be used to draw
conclusions. If it does not demonstrate the expected
it is ignored, and may then be plowed over to be run again.
III. SOME BASIC CONCEPTS IN STATISTICS
The statistical analysis of results is beyond the scope of
paper, but we must understand some basic concepts if we want
be able to interpret a statistician's analysis of our
The three concepts described briefly here are the normal
distribution, the null hypothesis, and the significant
A. The Normal Distribution
Assume that there is a large amount of some crop that is
under uniform conditions and harvested in plots of 100
feet. The yields recorded for each of these plots probably
vary from a very low figure to a very high figure. Most of
plots will yield close to a middle figure. As we move away
this median to either a higher or lower yield figure, we
find successively fewer plots. If the yield is plotted
the number of plots giving a particular yield, the familiar
bell-shaped normal curve will result. (see figure 1)
If the same crop is grown under identical conditions with
addition of a fertilizer treatment, there will still be a
range of yields for the 100-square-feet plots. However, the
entire curve will have shifted somewhat to the right. (see
Note that the two curves overlap in the crosshatched area;
plots may yield the same with and without fertilizer. If
a small number of the fertilized plots were measured, it is
possible that all or most of them would fall in this shaded
area. We would not know from our measurements whether the
fertilizer had really increased the yield.
The purpose of proper experimental design is to allow us to
determine whether the treatments have actually shifted the
normal curve, or whether the effect we observe is simply due
chance. This brings us to the next concept.
B. The Null Hypothesis
The statistician begins the analysis by assuming that the
treatments had no effect, and that any effect observed was
simply to chance. This assumption is known as the null
hypothesis. If we flip a coin and get "heads" four
times in a
row, we assume this to be due to chance and not because of
special quality of the coin.
Next, the statistician processes the data to determine the
validity of the null hypothesis. He or she may reject the
hypothesis, deciding that the observed effect of the
was significant, and probably not due to chance. Or, he or
may accept the null hypothesis, concluding that the observed
effect of the treatment was probably due to chance.
C. The "Significant Difference"
The term significant will be found in the results of many
experiments. This may also be indicated by an asterisk (*)
by the phrase "significant at the 5% level." These
that the statistician has determined that there is only a
percent chance that the observed difference was due to
If the results are found "highly significant,"
indicated by a
double asterisk (**) or by the phrase "significant at
level," this indicates that there is only a one percent
that the observed effect of the treatment was due to
This discussion indicates that a single experiment, no
how carefully designed and executed, cannot conclusively
"prove" that a treatment has a significant effect.
This is why
experiments should be repeated.
HOW TO PERFORM AN AGRICULTURAL EXPERIMENT
I. PRELIMINARY RESEARCH
Good preliminary research, including a search of the
literature and interviews of experienced persons, will save
great deal of trouble later. The experimenter should not be
afraid to ask for help now; help may be of no use once the
experiment has been laid out. The preliminary research
cover the following points:
(1) A careful study
of the crop should be made. The local soil
studied in fertilizer and irrigation experiments.
For pest control
experiments, information on the
life cycle of
the pest should be obtained.
(2) Economic factors
should be studied, especially if a new
crop is being
introduced. Will treatments affect the
market for this
crop? What is the cost of treatments?
(3) Has this
experiment been performed already? Quite likely,
experiment has been carried out. Were the
and do they affect the planned experiment?
experiments been carried out in other
The preliminary research should be recorded so that it may
included in the final report.
II. DESIGNING THE EXPERIMENT
In any experiment, errors are introduced by factors beyond
control of the experimenter: soil heterogeneity, plant
(due to genetic variability), plant competition within
and between plots, variation in the moisture content of
plants, climate variations (when experiments are run for
more than one year) , and the size and shape of plots. Such
errors cannot be eliminated, but they can be reduced,
by the replication of treatments and use of random
careful selection of treatments and location, and the
proper design of plots,
Replication is the repetition of a treatment several times
obtain a mean value or yield. In field experiments, a single
replicate generally is planned to contain one plot of each
treatment in a rather compact block. Replication is
by repeating blocks. A nonreplicated trial is not an
The number of replications depends upon the degree of
desired and the degree of soil heterogeneity. Generally, two
replications is not enough. The American Society of Agronomy
suggests 3-6 replications for field plots. The small number
suffices where average rather than annual results are
For corn yields, 4-6 replications are often used. For small
nursery plots, 5-10 replications are recommended.
B. Random Distribution
Random distribution means that treatments are assigned to
in a random fashion, or are placed randomly within a block.
reason for doing this is to eliminate any bias that might
if we assigned treatments to plots with some kind of order
The random distribution procedure should be completely
It may be accomplished by flipping a coin, drawing cards
from a deck, or by using a specially prepared table of random
numbers, such as the one found in the appendix of this
1. The completely random design
This design results when treatments are assigned to a
determined number of plots. It is useful for some types of
treatments on animals, but is not an efficient design for
trials with plants. Its main advantage is its simplicity and
flexibility. Treatments are assigned to plots by drawing
from a deck, slips of paper from a container, or by using
table of random numbers in the appendix.
Example: A, B, and C represent three different levels of
tested on wheat. Four samples for each
three levels = 12 plots.
2. The random complete block
In this design, treatments are assigned at random within a
block, and the entire block is replicated (see Figure 4).
blocks should be kept as compact as possible, and the number
treatments as low as possible consistent with the objectives
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The main advantage of the random complete block design is
high reliability of the data obtained from it, and its
for demonstration (as seen in Figure 5).
Example: A-F are six different fertilizer treatments for
Note that each treatment occurs
once in each
block. Six treatments X five
= 30 plots.
3. The Latin square design
In this design, treatments occur once in each column and
in each row, and treatments are random in both directions
Figure 6). Thus, the Latin square removes variability in two
directions while the random complete block removes it in
one direction. The number of replications always equals the
number of treatments in a Latin square design. It is more
than the random complete block, but it becomes cumbersome
for more than eight treatments.
In Figure 6, columns and rows are first numbered from 1 to
and treatments are assigned to the plots in regular
order, simply rotating the order one place in each row or
In the middle square, we have the same square after the
have been rearranged by choosing at random the numbers at
heads of the columns.
In Step 3, we have now chosen the rows at random by the same
method. The procedure is completed. Note that in the
square, treatments appear only once in each row and column.
4. The split-plot design
This design is used to test two factors in combination. It
not the most precise design for this purpose, but is often
to facilitate physical operations. For example, some field
treatments, such as irrigation, are more conveniently
to relatively large strips through the experimental area. If
different dates of harvest are one of the factors being
it may be easier to harvest in strips through the experimental
area rather than to harvest a few feet of one row and then
across rows for another small harvest area.
There are many split-plot designs. They vary in precision.
possible, an experienced person should be contacted for
before one uses this design. The basic design involves
one factor to main plots that are arranged in random
blocks or in a Latin square. Assign to the main plots
those treatments for which you are willing to sacrifice
The treatments of the second factor are assigned at
random to sub-plots within each main plot.
Example: Planting dates and fertilizer treatments on
Three planting dates (main plots) X
fertilizer treatments (subplots) X three
= 36 plots.
C. Selection of Treatments
Many factors that influence the farmer's profit can be
as contrasting practices in an experiment. Rate of seeding,
date of planting, spraying and dusting treatments, fall vs.
spring plowing, method of seed bed preparation, surface vs.
furrow application of irrigation water, weed control by
vs. cultivation, fertilizer treatments, pasture grass-legume
mixtures, and crop rotations are only a few of the more
In selecting fertilizer treatment rates, it is desirable to
rates that differ by equal intervals, such as 20, 40, 60,
and 100 pounds of nitrogen per acre. We may have an idea of
what rate would be inadequate and what rate would be well in
excess of optimum. We should test the entire range,
two or three levels between the minimum and maximum. An
untreated control plot is not necessary in a fertilizer plot
where it is understood that the crop needs some minimum
of fertilizer to grow well. However, the demonstration value
any experiment will be enhanced if we designate a control
that represents the local practice.
In a factorial experiment, the effect of more than one
is studied. For example, we may study the effects of four
levels of nitrogen and three levels of phosphorus. This
give 3X4 or treatment combinations. You should try to keep
experiment simple, not studying too many factors at once.
D. Selecting the Location
This is a highly critical step in the performance of an
The most important consideration in selecting a location
is soil heterogeneity. It was formerly believed that
experimental field should contain many different soil types
be representative." This is a misconception. The soil
representative of that generally found in the area. However,
the land within the experimental area should be as uniform
possible with respect to topography, fertility, the subsoil,
and previous management.
The causes of soil heterogeneity are the following:
(1) Topography: hillsides may cause gullies and the washing
nutrients. Low spots or variation in the texture
of the subsoil
will cause plant variation.
(2) Variation in the moisture content.
(3) Variation in the penetration of irrigation water.
(4) Wide variation in available soil nutrients.
(5) Competition and shading from trees and hedgerows.
(6) Past use of the soil, including previous varietal and
and previous applications of organic
and crop refuse.
What steps can we take to reduce the soil heterogeneity?
(1) Select land with a slight (1-2%), uniform slope. Avoid
use of draws,
lowlands, and other irregularly shaped
pieces of land.
(2) Where previous trials have been run that might affect
one or more "blank trials" before experimenting.
A blank trial is
a single crop--preferably a
is grown as uniformly as possible over
the entire field
to "smooth out" soil variations.
(3) Place new plots at a right angle to previous plots.
(4) Select land at least 20-30 yards from trees, hedgerows,
(5) Record all information concerning the past history and
of the land and included it in the final
report. This will
assist others in interpreting the
E. Plot Size and Shape
1. Plot size
In most local experiment stations or schools where land is
limited, the size and shape of the plot is a matter of
However, there are several considerations to take into
There are two basic plot sizes: (a) nursery plots, cared for
hand, which often have multiple short rows 10-22 feet long;
(b) field plots, adapted to the use of standard farm
Larger plots commonly are used for corn, sugar beets, and
rather than for small grains. Small plots may be necessary
where many varieties or strains are being tested, where the
amount of seed of a new variety is limited, or where funds
Researchers generally agree that an increase in plot size
reduce the error for plots up to about 1/40 acre (100 square
meters). Above that size, the decrease in error is less than
would be provided by an increase in the number of
Small plots are more variable due to (a) fewer plants,
losses in harvest or errors in measurement, and (c)
and greater border effects.
2. Plot shape
Plot shape generally makes no difference. Relatively long,
narrow plots should have their long dimension facing in the
direction of the greatest soil variation so as to overcome
There are two other practical considerations in plot shape.
First, plots should be sufficiently wide to allow border
to be removed or to minimize the importance of borders that
remain. Second, field plots should be of a shape and size to
accommodate farm machinery.
3. Suggested plot sizes and shapes for various crops
(from Field Plot
Technique by E. L. Leclerg, et al.
* Small grain: 3-4 rows X 10-20 feet (center rows
* Corn: 4-6 rows X 10-12 hills.
* Soybeans: 1-4 rows (2-3 feet apart) X 16 feet.
* Sorghum: 2-4 rows X 30 feet (center rows harvested in 3
4 row plots).
* Alfalfa: 7 feet X 60 feet (center five feet harvested with
a mower); 5-8
drilled rows 7" apart with a 12-14" alley
rows; 3-5 drilled rows 12" apart with an 18"
alley, and the
entire plot harvested.
* Sugar beets: four rows (20-24" apart) X 30-60 feet
thinned to 12"
apart in row)
4. Border rows and guard areas
When there is competition between adjacent rows of different
varieties, especially where they differ in growth habits,
serious error may be introduced. In semi-arid or sub-humid
areas where plants compete for water, small grain yields are
greatly affected by plant competition. For this reason,
row plots are not used. With many crops, 3-5 row plots are
grown, but the two outside rows are not harvested for yield.
Where alfalfa rows are spaced 7" apart, interplot
is a serious factor. If alleys between plots are widened to
14", border rows should still be removed because the
itself may allow border rows to grow more vigorously than
plants on the inside rows.
Fertilizer application often requires the use of machinery,
the flow of such fertilizer may not be precisely controlled
the ends of the field. Therefore "guard areas" 1-2
feet wide at
the ends of the plot are thrown out.
III. EXECUTION OF THE EXPERIMENT
A. How to Lay out a Right Angle
If the corners of the plots are not laid out at exactly 90
degrees, plots will cover a different area than we imagine
do. The following procedure is based on the fact that a
triangle with sides in a 3:4:5 ratio forms a perfect right
* 50-foot cloth tape measure, heavy string, or wire marked
30, 40, and 50
(1) Lay out a baseline with stakes and string. The length of
this line will
equal the desired width of the total plot.
Place two stakes
(A and A') as corner posts, as shown in
Connect A and A' with string.
(2) Place a third stake (B) next to the string exactly 40
(3) Have a co-worker hold the end of the tape on corner
A while you draw an arc with a 30-foot
radius. You should
extended tape to draw this arc across the
the side boundary will pass.
(4) Have the coworker hold the end of the tape at stake B.
same procedure as in step (3), draw an arc
with a radius of
50 feet, as shown in Figure 9-B. Place a
stake (C) where
the two arcs cross.
(5) Tie a string from stake A to stake C. This forms a right
angle at A (see
Figure 9-C). Now repeat the process at A'.
B. Labeling and Mapping
Accurate mapping and labeling is a simple procedure that is
crucial for a successful experiment. For example, if someone
pulls up your marker stakes before the experiment is
and you have made no map for your records, the experiment
You must draw a map because field markers often are
by weather or tractor drivers. The map should refer to
structures, such as fence posts, standpipes, building
corners, etc. You should be able to locate each separate
exactly, even if all the stakes, strings, and labels are
removed from the field. Also at this stage, the planned
should be listed and described. The map should indicate
which treatment each plot receives.
Field markers should be written in grease pencil, which will
not wash off in the rain or by irrigation water. Stakes may
used to label plots; cardboard tags often are used in
Make sure your application, the field markers, and the map
agree at the time treatments are applied.
C. Uniform Application
Failure to apply treatments uniformly is a very common
that decreases the value of the experiment. Great care
be taken to insure that fertilizer, pesticides, seed
etc., are applied uniformly over the plot, as specified.
Application equipment should be cleaned between trials.
must be swept out when different varieties are being
If more than one worker is applying treatments, do not have
same worker apply the same treatment over more than one
Do not inadvertently add factors. For example, when
is side-dressed on a row crop, the shoes on the applicator
prune some of the roots, and this will affect plant growth.
proper untreated check would consist of a plot through which
the fertilizer rig had been pulled without the material.
soaked in a chemical should be compared with seed soaked in
water, not with dry seed.
Carefully weigh all the materials used, if so required. Calibrate
application equipment to make sure you are putting on the
amount you think you are. Fertilizer elements should be
several weeks before the application to allow time for any
chemical reactions to take place.
Obtain a uniform stand. Small grains will tiller--or put
new shoots--where adjacent plants are missing, but corn and
many row crops will not "fill in" empty areas. One
to plant thick, then thin down to the desired stand.
Uniform care of plots is important. Weeds greatly influence
crop yields and should be removed early in the trial.
IV. MEASURING AND RECORDING THE RESULTS
Considerable time and expense has been spent thus far, yet
experimenters fail in the end because they measure and
the results improperly. The experimenter may take
at the wrong time. Or he or she may take measurements at the
right time, but fail to put all results in numerical terms.
or she may measure at the right time, and do so in numerical
terms, but fail to measure all the affected attributes. Or
experimenter may do all these things correctly, but not
the results in a simple, complete form.
A. When Should Measurements be Taken?
Different varieties mature at different times, and therefore
should not all be harvested at the same time. The
must watch closely and harvest each variety as it matures.
or she must record the total days to maturity for each
The rate at which results are reached is sometimes
For seed germination, both the rate of emergence and the
of seeds germinating should be recorded.
B. What Should be Measured?
This is an extremely important question, one not adequately
considered by inexperienced experimenters. In some
workers may simply harvest and weigh the crop with no
for other factors that are important on the market, and
which may have been affected. The market and nutritional
of the product must always be kept in mind. Even at a local
experiment station or school where there is no sophisticated
measuring equipment, there are many attributes that can be
measured. For example, fertilizer treatments on tomatoes may
affect not only the total yield, but also the time to
the color, the size and shape, and the susceptibility to
diseases. For corn, the number of ears should be counted,
and--if facilities are available--the moisture percentage
measured for a sample of ears that represent all sizes, with
kernels from one or two rows on each ear.
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The following are other attributes of field and
crops that might be measured:
* Sugar content
of sugar beets
gravity of potatoes
* Grade of
* Oil and protein
content of soybeans
content of sweetclover
percentage and milling quality of oats
* Ginning and
fiber properties of cotton
* Pithiness of
In short, when deciding what to measure, always keep in mind
the value of the product on the market.
C. Put All Observations in Numerical Terms
Many attributes of quality do not readily lend themselves to
measurement in numerical terms. For example, we may want to
measure the amount of insect damage on crop leaves after
treatments. It may seem easiest to judge damage as
"light," "moderate," and
"heavy." But unless we put everything
in numerical terms, a statistician cannot make use of our
In the case of disease or insect damage, a convenient
scale should be set up. For example, to measure potato scab,
set a scale ranging from 0 to 10. Zero represents a potato
completely free of scab, and 10 represents a potato entirely
covered with scab. In some places, standard scales have been
established--1-5 or 1-7--and photographs representing each
are used as a method of standardization. In general, the
following recommendations may be made.
(1) Try to design the scale so that observations are
is, the middle number is the most frequently
(2) There should be as many steps in the scale as an
(3) Where any individual judgment is involved in making
to avoid having more than one person
D. A Report Procedure
Research is a continuous process, even at the local level.
Single experiments seldom determine new farming practices;
results of several experiments have a cumulative effect. For
this reason and others, the written report of our experiment
must receive some attention. It must be complete, but not
complex. It must convey clearly and concisely what the
experimenter tested, under what conditions the test took
and the results. If the report is to be placed in a file
similar reports, there may already be a standard format. if
there is no sample format, the following is generally
(1) Title page. This should indicate clearly the nature of
experimenter's name, the date, and location
must be included.
(2) Introduction. This must include a review of the
background information, including all similar
carried out previously. The problem must be
(3) Procedure. This must include pertinent soil and climatic
careful description of the treatments, and
an explanation of
how the treatments were applied.
(4) Results. These should be given in both tabular and
form, with the
results of the statistical analysis shown
(5) Conclusion and recommendations. As a minimum, any
called for by the results should be mentioned.
(6) Appendix. This may include a plot map and the
APPENDIX: TABLE OF RANDOM NUMBERS(1)
To make random any set of ten items or less, begin at a
point on the table and follow either rows, columns, or
diagonals in either direction. Write down the numbers in the
order they appear, disregarding those that are higher than
numbers being made random and those which have appeared
in the series. If you wish to make random more than ten
numbers, pairs of columns or rows can be combined to form
digit numbers and the above process followed.
(1) Thomas M. Little, and F. J. Hills. Experimental Methods
Extension Workers. (Davis, California: University of
Agricultural Extension Service, 1966), p. 55.
Hopp, Henry. A Guide to Extensive Testing on Farms.
Foreign Agricultural Service, 1951.
Leclerg, E. L. , Leonard, W. H. , and Clark, A. G. Field
Minneapolis: Burgess Publishing Co., 1962.
Little, Thomas M., and Hills, F. J. Experimental Methods for
Workers. Davis, California: University of
Agricultural Extension Service, 1966.
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