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A difference-in-differences calculation subtracts the comparison group's change from the focal group's change. Causal interpretation needs assumptions such as comparable underlying trends and no other differential changes. These fictional historical data are only arithmetic practice.
Worked example
A rises from 20 to 30; B rises from 15 to 19. The change difference is 10 - 4 = 6, not proof by itself that a policy caused 6.
