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Multiple treatment interference is a threat to the internal validity of a group design. A problem occurs when participants in one group have received all or some of a treatment in addition to the one assigned as part of an experimental or quasi-experimental design. In these situations, the researcher cannot determine what, if any, influence on the outcome is associated with the nominal treatment and what variance is associated with some other treatment or condition. In terms of independent and dependent variable designations, multiple treatment interference occurs when participants were meant to be assigned to one level of the independent variable (e.g., a certain group with a researcher assigned condition) but were functionally at a different level of the variable (e.g., they received some of ...

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