  After a rich history in medicine, randomisation control trials both simple
and complex are in increasing use in other areas such as web-based AB testing
and planning and design decisions. A main objective is to be able to measure
parameters, and contrasts in particular, while guarding against biases from
hidden confounders. After careful definitions of classical entities such as
contrasts, an algebraic method based on circuits is introduced which gives a
wide choice of randomisation schemes.
