Attention deficit hyperactivity disorder management
There are two major medications derived from methylphenidate's isomers: a racemic mixture of half d-threo-methylphenidate and half l-threo-methylphenidate called methylphenidate (Ritalin, Concerta), and an enantiopure formulation containing just d-threo-methylphenidate called dexmethylphenidate (Focalin).
Rubin causal model
Rubin defines a causal effect:
Intuitively, the causal effect of one treatment, E, over another, C, for a particular unit and an interval of time from formula_1 to formula_2 is the difference between what would have happened at time formula_2 if the unit had been exposed to E initiated at formula_1 and what would have happened at formula_2 if the unit had been exposed to C initiated at formula_1: 'If an hour ago I had taken two aspirins instead of just a glass of water, my headache would now be gone,' or 'because an hour ago I took two aspirins instead of just a glass of water, my headache is now gone.' Our definition of the causal effect of the E versus C treatment will reflect this intuitive meaning."
According to the RCM, the causal effect of your taking or not taking aspirin one hour ago is the difference between how your head would have felt in case 1 (taking the aspirin) and case 2 (not taking the aspirin). If your headache would remain without aspirin but disappear if you took aspirin, then the causal effect of taking aspirin is headache relief.