The changing of treatments in a long-term trial

SUMMARY The changing of treatments during the course of an experiment is discussed both from the point of view of designing trials in which changes are anticipated and of modifying trials that have fulfilled their original purpose. Cases are distinguished according to whether the interactions of original and new treatments are or are not important and recommendations are made for each case. We are indebted to Miss J. M. S. Thom for preparing the diagrams fot publication.