sentences:  Normally patients would be allocated to a treatment group randomly and while this maintains a good overall balance, it can lead to imbalances within sub-groups.For example, if a majority of the patients who were receiving the active drug happened to be male, or smokers, the statistical usefulness of the study would be reduced.The traditional method to avoid this problem, known as blocked randomisation, is to stratify patients according to a number of factors (e.g.