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One consideration in research design is the nature of the variables that are being manipulated and measured, which can be treated as fixed or random. The independent, or treatment, variable in an experiment is referred to as a factor, because it is a controlled variable of which the levels are set by the experimenter. A fixed factor is one for which data has been gathered at all levels of interest. That is, if a researcher is interested in three particular levels of a factor, such as three specific medical treatments, then all three of those treatments have been tested in the experiment as conditions. Accordingly, a fixed factor is assumed to be measured without error since there is no variance between the variables measured in ...

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