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RS- 6
Principle of Experimental Designs
Experimental design refers to the rules regulating the assignment of treatments to the
experimental plots. Experimental design allows valid comparisons among treatments and it
controls the principal source of variation in the field experiments. A proper experimental design
must include proper blocking of main plots, replication and randomization.
Blocking
Blocking refers to the assignment of a group of plots or treatments to a block of land with
relatively homogeneous soil. It is one of the simplest and most effective ways of coping with soil
heterogeneity. Such blocking is required for the field crops experiment.
Effect of blocking
The variation among the blocks can be removed from the experimental error through blocking;
thus error is reduced and the precision of an experiment is increased. The larger the differences
among blocks, the greater the reduction in the experimental error. Hence, proper blocking should
produce large differences among blocks, leaving plots within a block more homogeneous.
How to block
When the fertility pattern of the experimental field is known, orient the blocks so that soil
differences between blocks are maximized. For example, for a field with an unidirectional
fertility-gradient along the length of the field, blocking should be made across the width of the
field, that is cutting across or perpendicular to the gradient. If fertility gradient is not known,
avoid using long and narrow blocks. Instead, use blocks that are as compact or nearly square as
possible, since plots that are closer can be expected to be more alike than those that are far apart.
Conduct all management operations and data collection “on a per-block basis” to control any
variation that may occur in the management and operation processes as well as in the data
collection. In other words, whenever a source of variation exists, attempt to have the major
portion of the variation separated by blocks. For instance, when an operation (for example,
application of treatments, measurement of data) can not be completed for the whole experiments
in 1 day, at least complete the work for all plots in a block. In this way, the difference, if any,
from day to day can be controlled by “blocking” If the work is to perform within a day, then
assign each operator to each of the block of the experiment.
Replication
A number of times a complete set of treatments repeated in an experiment is called replications.
Effect of replication
Replication is required in an experiment to provide a measure of experimental error. Moreover,
one of the simplest means of increasing precision is increasing the number of replications.
However, too many may not be justified. At the most 3 replications are justifiable in Bhutan.
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How to determine the number of replication
The number of replications you need depends on the magnitude of experimental error that is
likely to occur in the experiment and the degree of precision you want.
Randomization
Randomization is one of the basic principles of experimental design. It makes valid estimate of
experimental error which is essential for comparing treatments. It is a procedure for allocating
treatments so that each experimental plot has the chance of receiving any treatment.
How to randomize
The process of randomization can be done with a table of random number or by drawing lots.
The use of both methods is shown below for randomization and lay out of plots in a randomized
complete block design with six treatments and having four replications.
Randomization of treatments using Random number table.
Step 1. Locate a starting point in a table of random (Fig.3). Do this by pointing to a number in
the table with your eyes closed. Use this number as the starting point. On a piece of paper write
six consecutive three digit numbers beginning at the starting point and reading to the right or
downward. Picking up six numbers for six treatments For example, starting point is 78548 then
reading downward vertically get 6 three digit numbers.
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Three digit No. Sequence
548
1
603
2
621
3
428
4
939
5
911
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Step 2. Rank the selected numbers from the smallest to the largest
Three digit No.
939
911
621
603
548
428
Sequence
1
2
3
4
5
6
Rank
5
6
3
2
1
4
Step 3. In the third, use the rank as the treatment number and use the sequence in which the
treatment numbers occurred as the plot number in the block to which the corresponding treatment
will be assigned. Thus in this example, assign treatments no. 6 to the first plot, treatment No. 5
to the second, treatment no. 1 to the third, treatment 2 to the fourth, treatment no. 4 to the fifth
and treatment no. 3. to the sixth.
This is the lay out of the first block
Plot
Treatments
1
treatment no 5
2
treatment no 6
3
treatment no 3
4
treatment no 2
5
treatment no 1
6
treatment no 4
Step 4: Repeat the first three steps for Block II, then for Block III, and finally for Block IV.
Randomizing the treatments using the drawing lot method.
Step 1: Write the number 1 to 6 on six equal-sized pieces of paper, if you have six treatments to be
tested. Fold and place them in a box or in your Gho pouch.
Step 2: Shake the box or your Gho pouch to ensure thorough mixing of the slips of paper. Write down
the number and without returning the slip of paper already taken, pick up the second slip of paper. Repeat
the process until all six pieces of paper are taken out.
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For example, the numbers written on the slips of paper may appear in this sequence:
Sequence
Paper no.
1
4 (1st draw)
2
2 (2nd draw)
3
1 (3rd draw)
4
5 (4th draw)
5
6 (5th draw)
6
3 (last draw)
Step 3. Assign the six treatments to the six plots in the first block by using the number on paper as the
treatment number and the sequence as the plot number in the blocks.
The lay out of the first block is
Plot no
Treatments
1
treatment no 4
2
treatment no 2
3
treatment no 1
4
treatment no 5
5
treatment no 6
6
treatment no 3
Experimental Plot
Experimental plot refers to the unit on which random assignment of treatments
are made.
Treatment 1
Treatment 3
Treatment 2
Treatment 3
Treatment 1
Treatment 4
Treatment 2
Treatment 4
Treatment 3
Treatment 4
Treatment 2
Treatment 1
Experimental unit.
Plot Size
Size of the plot refers not only to the harvest area but to the whole unit
receiving the treatment.
Border
Harvest Area
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Plot Shape
Shape of the plot refers to the ratio of its length to its width. Example, length is
5 meters and width is 2 meters or length is 2 meters and width is 2 meters.
5 m X 2 m
Orientation of Plots
The orientation of plots, on the other hand, refers to the choice of direction
along with the lengths of the plots to be placed. The orientation of plots
naturally is not defined for square plots.
Effects of plot size, shape, orientation.
The size, shape and orientation of a plot can greatly affect the
magnitude of experimental error in a field trial. Too small plots may give
unreliable results and unnecessarily large plots waste time and resources. In
general, experimental error decreases as plot size increases, but the reduction
is not proportional.
Plot size not only affects variability but may also bring about bias in the
experimental results. Plots should be wide enough to permit the removal of
border rows when necessary (border effects).
For a specified area of land, the number of replication decreases as the plot size
is increased. Consequently a gain in precision from increased plot size is
accompanied by a loss of precision from reduced number of replications. But in
general, as long as the minimum plot size is reached, a larger increase in
precision can be expected with an increased number of replications.
How to choose plot size, shape, orientation
In general, field experiment plots usually range from 8 to 25 m2. Whatever size
and shape of plot you choose, make sure that an area not smaller than 5 sq
m, free from all types of competition and border effects, is available for
harvesting and determining plot yield. Consider the following when choosing
plot size and shape.
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The type of experiment can dictate the size and shape of plots for ease of
operations. For example, the fertilizer trials require larger plots than variety
yield tests. Irrigation studies may require even larger plots. In insecticide or
herbicide trials where the chemicals are to be sprayed, the width of the plot
may be governed by the range of coverage of the sprayer used.
Border effects
In experiments where border effects (Fig. 9) might be appreciable, square plots
are desirable because they have maximum perimeter for a given plot size. In
varietal yield tests where varietal competition is expected, plots with at least six
rows should be used to allow exclusion of one row on each side of the plot,
thus leaving four center rows for harvest.
Variety A
Variety B
Here variety A and variety B have acted alike
Site selection is choosing an appropriate place for conducting a trial in proper
manner.
To establish a trial, selection of site is very important. The site or the area used
for experiments should be as uniform as possible to reduce experimental error.
The selection of site is done by using the following criteria.
1. Problem area
2. Potential area
3. Topography of the area.
4. Climatic condition.
5. Status of the soil.
6. Cropping pattern.
7. Standing crop, livestock, timber trees condition.
8. Irrigation sources.
Example of the criteria
Problem: Crop diseases; Potential: Apple could be grown; Topography: Is the
field terraced; Climate: Suitable for crops; Status of soil: Is it fertile; Cropping
pattern: The cropping pattern of the field; Standing crop, livestock, timber trees
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condition; Irrigation sources: Is water enough for paddy crop. For example, if
you are conducting a trial on crops in a farmer’s field, then you should talk to
the farmer about his fields and ask for a good terrace which has uniform
fertility in order to reduce experimental error. Try to work with good farmers
because if crops and fields are not managed well, it will be difficult to get good
results. For example, if there is lot of weeds on the terrace, then it is bad
management. In some experiments, the farmer is asked to manage the trial for
irrigation and application of fertilizers.
In other experiments, the person setting up the trial is asked to look after it. If
you are managing the trial, choose a site with easy access for trail management
and monitoring and to avoid carrying heavy equipment for long distances. If
the trial is on water management, check for good irrigation source.
Finding out the Area of the selected Site
After finding out a suitable site, then work out the total area required by your
trail. The area requirement is the number of treatments by the number of the
replications and then by the plot area. An example, when conducting a trial on
Rice varieties, the followings are the requirement.
Plot area
10 m2. (5 x 2)
No. of Treatments
3
No. of replications
3
Irrigation channel between replication
0.5 m
Space between varieties
0.3 m
Path way around the periphery
1.0 m
Therefore, the total area required by the experiment is 17.6 meters by 9 meters.
Measure the selected site where the experiment is going to be set up. Measure
the four sides of the site, using the shortest length and shortest width. This will
give the maximum area available within the selected site. If the available area
is not enough for your trial, either change the site or reduce the trial area. But
reducing of trial plot size will not have good results.
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