  Crossover clinical trials can provide substantial benefits by eliminating
inter-patient variation from treatment comparisons and by allowing multiple
observations of each patient. They are particularly useful when sample sizes
are necessarily small. These advantages proved particularly valuable in an
assessment of clot prevention in children undergoing haemodialysis. Only small
numbers of children are treated at any given time in any single unit, but each
patient is obliged to attend two or three times each week, suggesting the use
of a crossover trial with many periods. Standard crossover trials described in
the literature a) typically have fewer than 10 periods and b) are based on a
model of questionable applicability to this study. This paper describes the
derivation of an optimal crossover trial with 30 periods which was used to
compare the treatments using nine patients.
