When conducting between-subjects designs, larger sample sizes are needed by default in order to compare independent groups. Larger sample sizes are also needed in between-subjects comparisons to yield the most precise and accurate inferences related to differences between groups.
In some between-subjects designs, researchers will collect more "control" observations than "case" observations to improve upon statistical power. A ratio of 1:1, 2:1, 3:1, or 4:1 for controls to cases is perfectly acceptable in case-control and retrospective cohort designs. This is especially useful when the cases are very rare or few observations can be collected.
Regardless, between-subjects designs will always decrease statistical power and increase the needed sample size.
In some between-subjects designs, researchers will collect more "control" observations than "case" observations to improve upon statistical power. A ratio of 1:1, 2:1, 3:1, or 4:1 for controls to cases is perfectly acceptable in case-control and retrospective cohort designs. This is especially useful when the cases are very rare or few observations can be collected.
Regardless, between-subjects designs will always decrease statistical power and increase the needed sample size.
Between-subjects designs decrease statistical power and increase the needed sample size.
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