In a laboratory experiment with mice, the 5 mice to receive the treatment were selected by the investigator by plunging his hand into the cage containing 10 mice and catching them one at a time. This method may appear to be random but perhaps the mice chosen are slower, friendlier or even sicker than the mice who were not caught. This would result in treated group being slower or sicker than the control group. Randomisation would give each mouse an equal chance to be chosen in the treatment group. The steps in randomisation usually consist of assign labels to the experimental units use a method to select labels at random e.g. using random number tables or computer generated random numbers.
Most statistical analyses involve comparisons of groups.
If the investigator randomises the subjects to the respective groups and hence the intervention, the study is called an experiment. If subjects are not randomised, it is an observational study . Randomisation is obtained by use of a coin or a table of random numbers. Cancer Survival in South Australia. This is an example of a retrospective case-control study .