### Judea Pearl on Potential Outcomes

I recently attended a seminar presentation by Professor Tom Belin, (AQM RAC seminar, UCLA, November 30, 2012) who spoke on the relationships between the potential outcome model of Neyman, Rubin and Holland, and the structural equation and graphical models which I have been advocating since 1995.

In the last part of the seminar, I made a few comments which led to a lively discussion, as well as clarification ( I hope) of some basic issues which are rarely discussed in the mainstream literature.

Below is a concise summary of my remarks which I present to encourage additional discussion, questions, objections and, of course, new ideas.

Judea Pearl

__Summary of my views on the relationships between the potential-outcome (PO) and Structural Causal Models (SCM) frameworks.__

Formally, the two frameworks are logically equivalent; a theorem in one is a theorem in the other, and every assumption in one can be translated into an equivalent assumption in the other.

Therefore, the two frameworks can be used interchangeably and symbiotically, as it is done in the advanced literature in the health and social sciences.

However, the PO framework has also spawned an ideological movement that resists this symbiosis and discourages its faithfuls from using SCM or its graphical representation.

This ideological movement (which I call “arrow-phobic”) can be recognized by a total avoidance of causal diagrams or structural equations in research papers, and an exclusive use of “ignorability” type notation for expressing the assumptions that (must) underlie causal inference studies. For example, causal diagrams are meticulously excluded from the writings of Rubin, Holland, Rosenbaum, Angrist, Imbens, and their students who, by and large, are totally unaware of the inferential and representational powers of diagrams.

Formally, this exclusion is harmless because, based on the logical equivalence mentioned above, it is always possible to replace assumptions made in SCM with equivalent, albeit cumbersome assumptions in PO language, and eventually come to the correct conclusions. But practically, the exclusion forces investigators to articulate assumptions whose meaning they do not comprehend, whose plausibility they cannot judge, and whose statistical implications they cannot predict.

The arrow-phobic exclusion can be compared to a prohibition against the use of ‘multiplication’ in arithmetics. Formally, it is harmless, because one can always replace multiplication with addition (e.g., adding a number to itself n times). Yet practically, those who shun multiplication will not get very far in science.

The rejection of graphs and structural models leaves investigators with no process-model guidance and, not surprisingly, it has resulted in a number of blunders which the PO community is not very proud of.

One such blunder is Rosenbaum (2002) and Rubin’s (2007) declaration that “there is no reason to avoid adjustment for a variable describing subjects before treatment”

http://www.cs.ucla.edu/~kaoru/r348.pdf

Another is Hirano and Imbens’ (2001) method of covariate selection, which prefers bias-amplifying variables in the propensity score.

http://ftp.cs.ucla.edu/pub/stat_ser/r356.pdf

The third is the use of ‘principal stratification’ to assess direct and indirect effects in mediation problems. which lead to paradoxical and unintended results.

http://ftp.cs.ucla.edu/pub/stat_ser/r382.pdf

In summary, the PO framework offers a useful analytical tool (i.e.. an algebra of counterfactuals) when used in the context of a symbiotic SCM analysis. It may be harmful however when used as an exclusive and restrictive subculture that discourages the use of process-based tools and insights.

Additional background and technical details on the PO vs. SCM tradeoffs can be found in Section 4 of a tutorial paper (Statistics Surveys)

http://ftp.cs.ucla.edu/pub/stat_ser/r350.pdf

and in a book chapter on the Eight Myths of SEM:

http://ftp.cs.ucla.edu/pub/stat_ser/r393.pdf

Readers might also find it instructive to compare how the two paradigms frame and solve a specific problem from start to end. This comparison is given in Causality (Pearl 2009) pages 81-88, 232-234.

I agree.

PO and SEM are two different languages for the same thing and

anyone who wants to understand causality does themselves a disservice

if they are not conversant in both.

Larry

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