untitled

wheat has no effect on the density (or rate of increase) of quail. What is tested, how-

ever, is the statistical null hypothesis that the difference between treatments is not

significantly greater than the difference between replicates within treatments. If the

experiment rejects that null hypothesis we accept as highly likely the alternative hypo-

thesis that supplementary feeding affects the dynamics of quail populations living in

conditions similar to those of the populations being studied.

This design tests the effect of only one factor (WHEAT) and evaluates it at only two

levels (no wheat and some wheat). Note that the diagram of design logic calls for

two replicates at each level. The diagram of design layout shows that the treatments

are interspersed: thus we do not have the zero treatments (i.e. controls) bunched

together in one region and the wheat-added treatments in a second region.

One factor, several levels

This design (Fig. 16.2) is similar to the last, with the difference that the effect of

supplementary feeding with wheat is evaluated at four levels: 0, 30, 70, and 250 kg /ha

of wheat distributed each month. It allows an answer to two questions: first, whether

supplementary feeding has any effect at all upon the density of the quail; and

second, whether that effect varies according to the level of supplementary feeding.

An answer to the second question allows a cost–benefit analysis on the optimum level

of supplementary feeding. Treatment replication and interspersion of treatments is

maintained.

Two factors, two or several levels per factor

In this design (Fig. 16.3) the effect of supplementary feeding on quail density is

evaluated in tandem with an evaluation of a second factor, the provision of rock

salt.

Although the two factors could have been evaluated by two separate experiments

there are large advantages in combining them within the same experiment. It pro-

vides an answer to a question that might prove to be of considerable importance: do

the two factors interact?

276 Chapter 16

Factor: WHEAT (4 levels)

Response variable: Density or rate of increase of quail

Design logic

Rep

Rep

Rep

Rep

Design layout

Rep

Rep

Rep

Rep

0 WHEAT 30 WHEAT 70 WHEAT 250 WHEAT (kg / ha)

0

WHEAT

250

WHEAT

30

WHEAT

70

WHEAT

70

WHEAT

30

WHEAT

0

WHEAT

250

WHEAT

Fig. 16.2One-factor

experimental design

where the factor has

more than two levels.

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