Difference between Random Selection and Random Assignment

Random selection and random assignment are commonly confused or used interchangeably, though the terms refer to entirely different processes.  Random selection refers to how sample members (study participants) are selected from the population for inclusion in the study.  Random assignment is an aspect of experimental design in which study participants are assigned to the treatment or control group using a random procedure.

A study using only random assignment could ask the principle of the school to select the students she believes are most likely to enjoy participating in the study, and the researcher could then randomly assign this sample of students to the treatment and control groups.  In such a design, the researcher could draw strong conclusions about the effect of the intervention but could only make limited inferences about whether the effect applies to the entire population.

A study using only random selection could randomly select students from the overall population of the school, but then assign students in one grade to the intervention and students in another grade to the control group.  While the data collected from this sample could be used to make inferences about the population of the school, the lack of random assignment limits the conclusions that can be made about the intervention’s effect.

Random selection is thus essential to external validity, or the extent to which the researcher can generalize the results of the study to the larger population.  Random assignment is central to internal validity, which allows the researcher to make causal claims about the effect of the treatment.  Nonrandom assignment often leads to non-equivalent groups, meaning that any significant differences might be a result of the groups being different at the outset rather than different as a result of the treatment.  The consequences of random selection and random assignment are clearly very different, and a strong research design may employ both to strengthen both internal and external validity.

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