Caption: Random Assignment

Text: Randomized assignment means that all subjects have an *equal chance* of being allocated to any of the available treatment options. To be effective, It must be done by a method that is unpredictable. One can use published tables of random numbers and simply assign subjects based on the next number listed on the table, or one can use a random number generated by a computer, such as the random number function in Excel. The unpredictability means that, if a sufficiently large number of subjects are randomly assigned to treatment groups, the groups will have similar distributions of all characteristics. As a result both known and unknown confounders will tend to be equally distributed among the study groups. By avoiding an imbalance in other risk factors, the estimate of association is less likely to be influenced by confounding.