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 . Confounding A potential problem with observational studies is that there may be factors which differ between the comparison groups and which may affect the outcomes. If such a factor affects the outcomes it is called a confounding effect. You need to look out for such factors when you analyse data and make sure you present the results appropriately.