Comment: Which Ifs Have Causal Answers

I congratulate my friend Paul Holland on his lucidly clear description of the basic perspective for causal inference referred to as Rubin's model. I have been advocating this general perspective for defining problems of causal inference since Rubin (1974), and with very little modification since Rubin (1978). The one point concerning the definition of causal effects that has continued to evolve in my thinking is the key role of the stable-unit-treatmentvalue assumption (SUTVA, as labeled in Rubin 1980) for deciding which questions are formulated well enough to have causal answers. Under SUTVA, the model's representation foutcomes is adequate. More explicitly, consider the situation with N units indexed by u = 1, .. ., N; T treatments indexed by t = 1, . . . , T; and outcome variable Y, whose possible values are represented by Y," (t = 1, . . . , T; u = 1, ... , N). SUTVA is simply the a priori assumption that the value of Y for unit u when exposed to treatment t will be the same no matter what mechanism isused to assign treatment t to unit u and no matter what treatments he other units receive, and this holds for all u = 1, . . . , N and all t = 1, . . . , T. SUTVA is violated when, for example, there xist unrepresented versions of treatments (Y,u depends on which version of treatment t was received) or interference b tween units (Y,1 depends on whether unit u' received treatment t or t').