Treatments should be assigned so that each experimental unit has a known, often equal, probability of receiving a given treatment. This process, termed randomization, is essential because there are often sources of variation, known or unknown, which could bias the results. Most statistical packages for computers will produce random numbers within a specified range, which can be used in assigning experimental units to treatments. Some textbooks have tables of random numbers designed for this purpose. Alternatively, treatment assignments can be written on pieces of paper and drawn out of a bag or bowl for each experimental unit (e.g., animal or cage). If possible, the randomization method should ensure that there are predefined numbers in each treatment group. Note that the different treatment groups should be processed identically throughout the whole experiment. For example, measurements should be made at the same times. Furthermore, animals of different treatment groups should not be housed on different shelves or in different rooms because the environments may be different