For example, accountants may sample a firm's inventory to verify the accuracy of its entire records. An experiment randomly allocates the treatment on the experimental units or subjects in order to observe their response. 2 Experiments should compare treatments rather than attempt to assess a single treatment in isolation. Bias - the design of a study is biased if it systematically favours certain outcomes. Randomisation, Replication and Blocking The use of chance to allocate experimental units into groups is called randomisation .
Randomisation is the major principle of the statistical design of experiments.
Randomisation produces groups of experimental units that are more likely to be similar in all respects before the treatments are applied than using non-random methods. At the end of the study if the differences in the outcome variable between the two groups is too large to attribute to chance, then the difference is called statistically significant . This will be discussed in later sections. Another principle is that experiments with more subjects are more likely to detect differences than those with fewer subjects. Repeating an experiment on many subjects is called replication .