Aim of similar experimental units

In an experiment, we usually try to use experimental units that are as similar as possible. Variability between experimental units increases the variability in resulting response measurements and this 'noise' in the data makes it harder to detect the effect of the experimental treatments.

Therefore experiments involving mice often use strains of mice that are bred to be genetically very similar so that differences in a response variable between mice will be mostly caused by differences between the experimental treatments and very little by differences between the mice themselves.

In a similar way, all plants in an experiment will be grown from identical seed under as similar growing conditions as possible. (Other than the factor being controlled.)

Feed supplement and beef from cattle

Consider an experiment that is to be conducted to estimate how much a particular feed supplement increases the amount of beef that is obtained when cows are eventually killed. A herd of calves is available for the experiment and half will be given a standard diet with the other half getting the extra supplement. The response measurement is the weight of meat obtained from each cow when it is eventually slaughtered.

Before the experiment starts, the calves in the herd are not identical and we will exaggerate their differences for the sake of this illustration. The calves have different body weights and body weight has a strong influence on the weight of meat in the adult cows.

The diagram simulates a completely randomised experiment in which the feed supplement is given to half of the calves (selected at random). The horizontal axis shows the initial calf weights and the vertical axis shows the resulting weight of beef when the adult cows are slaughtered. The effect of the supplement is estimated by the difference in the mean weight of beef from the calves who were given the supplement and the control group receiving no supplement.

Click Accumulate then click Conduct experiment several times and observe that the estimate of the supplement's effect (on the right) is very variable — one run of the experiment does not give an accurate estimate.

The main reason for the inaccuracy in the estimates is the variability in the experimental units and, in particular, the variability in the initial weights of the calves. If the calves in the herd all had similar weights, the experiment would have resulted in a more accurate estimate of the effect of the supplement. Drag the slider to make the calf weights similar then repeat the simulated experiment several times. Observe that the estimate of the feed supplement's effect is now much more consistent.