Three Principles of Experimental Design

In a research study, the proper design and layout of experimental design is crucial to the success of the study. To accomplish this, the following principles must always be remembered and practiced:

a.Replication – repetition of the experiment on many experimental units

 Functions:

1.To provide for an estimate of experimental error which is used for tests of significance;

2.To improve the precision of the experiment by reducing the standard error of the mean;

3.To increase the scope of inference of the experiment; and

4. To effect control of error variance.

Precision – the closeness to one another of a set of separated measurements of a quantity

Accuracy – closeness to the absolute or true value of the quantity measured

Coefficient of Variation – indicates the variation within the data set

Factors that determine the number of replications:

1.      Uniformity of experimental units

2.      Experimental designs

3.      Degree of precision required

4.      Number of treatments

5.      Time allotment

6.      Cost and availability of funds or resources

b.    Randomization – use of chance to divide experimental units into groups

-          a process of assigning the treatments among the experimental units such that every treatment has equal chance of being assigned to any experimental unit

Functions:

1. To provide the random sample of observation.

2. To have a valid estimate of experimental error.

3. To remove the bias of assigning treatments continually to favorable experimental units.

4.To protect against the effects of factors that cannot be controlled perfectly.

Methods of Randomization:

1.      Draw lots

2.      Randomization softwares

3.      Table of random digits

4.      Random numbers

c.    Local Control – process of minimizing the experimental error

-          includes all practices or techniques used to minimize the experimental error which includes balancing, blocking, and grouping of experimental units

Techniques:

1.      Use an appropriate experimental design.

2.      Compare several treatments.

3.      Use appropriate experimental units; e.g. proper size, proper shape, etc.

4.      Proper handling of experimental units.

5.      Refinement of experimental units.