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A key element in experimental design is the independent variable, or factor. Two basic types of factors exist in the analysis of experiments: fixed and random. Unlike a fixed factor, in which all levels of interest have been measured, a random factor is one for which only a selection of all possible levels of a factor has been measured for analysis. In an experimental setting, this factor is an independent variable of which the levels manipulated in the study are intended to represent the broader population of possible levels (e.g., three media exposure levels selected to represent the full possible range of media exposure). Because of this, a random factor is assumed to be measured with some measurement error, since it must account for random ...

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