Introduction to Experimental Designs

By an experimental design, we mean a plan used to collect the data relevant to the problem under study in such a way as to provide a basis for valid and objective inference about the stated problem. The plan usually consists of the selection of treatments whose effects are to be studied, the specification of the experimental layouts, and the assignment of treatments to the experimental units and the collection of observations for analysis. All these steps are accomplished before any experiment is performed.

An experiment is planned to:

  1. Get maximal information for minimal expenditure in the shortest possible time;
  2. Avoid systematic errors;
  3. Evaluate the outcomes critically and logically;
  4. Ignore spurious effects, if any.

The following considerations go into the planning of an experiment:

  1. What is the experiment intended to do?
  2. What is the nature of the treatments or dependent variables and how are they to be estimated?
  3. How is the independent variable likely to affect the treatments or dependent variables?
  4. Are the factors to be held constant or varied? If varied, is this variation quantitative or qualitative?

The answers to these questions enable the experimenter to state their hypothesis precisely and to plan their experimental procedure in a more effective way.

There are two types of design: systematic and random. However the analysis of variance techniques is suitable to randomized designs only. The basic randomized designs are (i) Completely Randomized, (ii) Randomized Complete Blocks, and (iii) Latin Squares, which we shall discuss in the sections that follow.