Up to 20 treatments can be specified. It does not matter which boxes are left blank if they are not all needed. The program begins by sorting treatment names internally. Therefore, the way they are placed in the worksheet is immaterial to the final randomization plan.

The total number of subjects must be a multiple of the number of treatments. The randomization can be carried out within sub-blocks to insure against serious imbalance should the study be terminated prematurely. See Fleiss (1986, sec. 3.1). The number of subjects per sub-block must be a multiple of the number of treatments.

The seed for the random number generator (Wichmann and Hill, 1981, as modified by McLeod, 1985) is obtained from the clock of the local computer and is printed at the bottom of the randomization plan. If a seed is included in the request, it overrides the value obtained from the clock and can be used to reproduce or verify a particular plan.

Some Special Situations

To randomize subjects with respect to several experimental factors, use a set of labels composed of all possible combinations of the original factor levels. For example, if subjects are to be randomly assigned to one of two levels of dietary supplement (LOW/HIGH) and one of two types of oil (FISH/SOY), use a set of 4 treatment labels: "Low Fish", "High Fish", "Low Soy", "High Soy".

To generate random permutations, let the total number of subjects be the product of the number of permutations and treatments, and let the number of sub-blocks be equal to the number of permutations. That is, to obtain "n" permutations of "k" treatments, let the number of subjects be "nk" and the number of sub-blocks be "n". Each set of "k" treatments will be a random permutation of the original set.

To generate a plan with unequal numbers of subjects on each treatment, enter the labels in the worksheet in proportion to how they are to compose the plan. For example, to have twice as many subjects receive treatment A as treatment B, enter the label A twice and B once.

Fleiss, JL (1986). The Design and Analysis of Clinical Experiments. New York: John Wiley and Sons.

McLeod, A. Ian (1985), "Remark AS R58. A remark on algorithm AS 183. An efficient and portable pseudo-random number generator," Applied Statistics, 34, 198-200.

Wichmann BA and Hill ID (1981), "Algorithm AS 183. An efficient and portable pseudo-random number generator," Applied Statistics, 31, 188-190.