Randomization (or random allocation): method analogous to tossing a coin to assign patients to treatment groups (the experimental treatment is assigned if the coin lands heads and a conventional, control, or placebo treatment is given if the coin lands tails). Usually done by using a computer that generates a list of random numbers, which can then be used to generate a treatment allocation list.

Randomized controlled clinical trial (RCT)
A group of patients is randomized into an experimental group and a control group. These groups are followed up for the variables/outcomes of interest. Randomization is used in order to avoid any possibility of selection bias in a trial. The test to confirm that randomization has been successful is when different treatment groups have the same characteristics at baseline. For instance, there should be the same number of men and women, or older or younger people, or different degrees of disease severity.

Quasi-random allocation: a method of allocating participants to different forms of care that is not truly random; for example, allocation by date of birth, day of the week, medical record number, month of the year, or the order in which participants are included in the study (alternation).

A quasi-randomized trial uses quasi-random method of allocating participants to different interventions. There is a greater risk of selection bias in quasi-random trials where allocation is not adequately concealed, as compared with randomized controlled trials with adequate allocation concealment.

Stratified randomization
In any randomized trial it is desirable that the comparison groups should be as similar as possible as regards those characteristics that might influence the response to the intervention. Stratified randomization is used to ensure that equal numbers of participants with a characteristic thought to affect prognosis or response to the intervention will be allocated to each comparison group. For example, in a trial of women with breast cancer, it may be important to have similar numbers of pre-menopausal and post-menopausal women in each comparison group. Stratified randomization could be used to allocate equal numbers of pre- and post-menopausal women to each treatment group. Stratified randomization is performed either by performing separate randomization (often using random permuted blocks) for each strata, or by using minimization.

Bandolier. EBM glossary. Internet. Accessed on July 3, 2016.