🔀 Within-subject design vs. between-subject design
Terracotta supports both within-subject and between-subject experiment designs. In a between-subject design, half the participants get treatment “A” and half get treatment “B.” In a within-subject design, participants get multiple conditions at different times during the experiment - it might have half the participants get treatments "A-then-B," and the other half get "B-then-A".
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When each participant receives all treatments, there is less concern about the research causing inequities. For example, imagine that version B turns out to be better than version A for student learning. In a between-subject design, this might cause students who received version A to have worse outcomes, but in a within-subject design, all students received the same treatments, just staggered in time.
Because each participant in a within-subject design receives both treatments, each participant is almost like their own mini-experiment. In effect, the research study has more statistical power to infer differences between treatments A and B.
⚖️ Balancing the experiment
In order to have a balanced design, a research study needs to include as many experimental treatments as there are experimental conditions. Terracotta eliminates some of the complexities of organizing these sorts of experiments by organizing things around the idea of an “exposure set.”
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