The ultimate goal of clinical research, is to establish causality—to determine efficacy outcomes that are due to the drug and to measure their magnitude, and to determine adverse effects related to the drug.

How does one know whether an effect A (e.g. giving a particular drug at a particular dose) causes an event B? A number of conditions must be satisfied. First, A must precede B. Second, whenever A occurs, B must occur too. These, of course, are not sufficient, since both A and B could be caused by an effect C. In addition, therefore, a theory is required that links A to B. This requirement is the Achilles heel of 'causality', since all theories are necessarily tentative. In an experimental science such as pharmaceutical research, the second condition can be established by conducting an experiment both when effect A is absent and when effect A is present, while all other conditions remain unchanged. If B requires the presence of A, then B is caused by A. However, if B is present regardless of A, then no causality is proven because B may be caused by an effect C that is present in both parts of the experiment.

In studying drug effects in humans, the controlled clinical trial is the preferred method to establish causality. In its simplest form, a controlled clinical trial is an experiment in human subjects in which some subjects are treated with an investiga-tional drug and some are not, while all other conditions remain the same for the two treatment groups. In this way, any differences in clinical outcome should be due only to the investigational drug (controlled clinical trials will be discussed in greater detail in the section on The Controlled Clinical Trial, below).

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