Starting with simple experiments
Experiments such as those described on the previous page are often complex. There are three aspects to this complexity.
We will build up to this complexity gradually, starting with relatively simple experiments.
Homogeneous experimental units and a single factor
The most fundamental aspect of all experiments is that we apply treatments to experimental units, so we initially concentrate on the treatment structure for experiments with simple experimental units; later chapters will consider other complications.
This chapter restricts attention to the simplest possible type of experiment in which:
This is more useful than it seems because the concepts and methods that are introduced in this chapter are the basis for design and analysis of more complex experiments in later chapters.
Rice yield and insecticides
Scientists investigating chemical control of brown planthoppers and stem borers in rice conducted an experiment to compare the effectiveness of six different foliar and granular insecticides. Twenty eight plots of land were used and the six different insecticides and a control treatment were each randomly applied to four of the plots. The table below shows the resulting yields of rice.
Grain yield, kg/ha | ||||
---|---|---|---|---|
Dol-Mix (1 kg) | 2,537 | 2,069 | 2,104 | 1,797 |
Dol-Mix (2 kg) |
3,366 | 2,591 | 2,211 | 2,544 |
DDT + γ-BHC | 2,536 | 2,459 | 2,827 | 2,385 |
Azodrin |
2,387 | 2,453 | 1,556 | 2,116 |
Dimecron-Boom |
1,997 | 1,679 | 1,649 | 1,859 |
Dimecron-Knap | 1,796 | 1,704 | 1,904 | 1,320 |
Control |
1,401 | 1,516 | 1,270 | 1,077 |
Lamb weight gain and hormones
An experiment was conducted to determine the effects of three different hormones, each at a single dose, on the weight gained by lambs. Sixteen lambs were used with four randomly allocated to each of the three hormone treatments and the remaining raised without any hormone added. The table below shows the weight gains of the lambs (pounds per animal per day).
Weight gain of lambs | ||||
---|---|---|---|---|
Control | 47 | 52 | 62 | 51 |
Hormone A | 54 | 65 | 75 | 59 |
Hormone B | 50 | 54 | 67 | 57 |
Hormone C | 57 | 53 | 69 | 57 |
Effect of copper on aquatic animals
A study was designed to determine the effect of copper concentrations in water on the lifetime of aquatic animals. Daphnia magna, small aquatic animals, were selected for the study since they have relatively short lifetimes.
Fifteen daphnia, each to be kept in separate containers, were available for the study. Five daphnia were randomly chosen for each of three copper levels for the water: no copper, 10 micrograms/litre and 40 micrograms/litre.
The lifetime (days) of each daphnia was recorded. One container was contaminated, so only 14 of the 15 measurements could be analysed. The experimental results are shown below:
Copper concentration | ||
---|---|---|
None | 20 µg/l | 40 µg/l |
60 90 74 82 |
58 74 50 65 68 |
40 58 25 30 42 |