Experimental designs

These are blueprints for the experiments. They define both the pattern for applying the treatment to the test units and how to measure the treatment's impact. Experimental designs fall into two broad categories:

1 basic (informal) designs

2 statistical (formal) designs.

The main difference between the two is that basic designs concentrate on measuring only the impact of the treatment, whereas statistical designs also measure the impact of other factors in addition to the treatment.

There are a number of basic designs that attempt to measure the effect of the treatment (Figure 9.3). The methods differ in terms of the approaches they employ to isolate the treatment's impact. Some of the more popular designs are now described (see Cochran and Cox4 for more designs).

FIGURE 9.3

Types of experimental design

The following symbols are used to help in presenting these designs: X = the period in which the treatment is applied

O = the observation periods before and/or after the treatment; subscripts are used to identify the time of each observation EG = the experimental group: the subjects receiving the treatment OG = control group: the subjects matched up with the experimental group, but not receiving the treatment

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