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The final choice of design for a clinical trial will depend on many factors, key amongst these are clearly the specific research question posed, the practicality of recruiting patients to such a design and the resources necessary to support the trial conduct. We shall discuss these and other issues pertinent to the design choice in later chapters. Nevertheless, we can catalogue the main types of design options available and these are listed in Figure 1.2. This gives a relative weight to the evidence obtained from these different types of clinical trial. All other things being equal, the design that maximises the weight of the resulting evidence should be chosen. For expository purposes, we assume that a comparison of a new test treatment with the current standard for the specific condition in question is being made.