When subjects are not randomly assigned, but rather come to the study as members of each condition a non-manipulated independent variable is being used.
Studies or researches are continuous and rigorous observation of certain field/ subject which is used to testify the presence of that subject and to get appropriate attention for gaining suitable knowledge about it. When certain parameters are used in studies, the probable effects of them are usually pre analyzed. However when some new parameter is added, it tends to alter the predictions in some way and hence their effects are not easily evaluated and so they are classified and non-manipulated independent variables because their source of origin are not clarified in due course of study. These studies are important part of experimental designs.
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