If you are at the point where you are deciding who from the population shall be your participants, you should be using:

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If you are at the point of deciding who shall be your participants, it is important to use random sampling to ensure that your sample is representative of the population and avoids any potential biases or confounding factors. Random assignment (B) is used once you have selected your participants to assign them to different conditions in your experiment. Factorial design (A) and Solomon's four-group design (C) are experimental designs that are used after the participants have been selected and assigned to conditions.

If You Are at the Point Where You Are Deciding