Causality in Time Series - ClopiNet

PopescuType III error prob. γ = P (︁ Ĥ a |H b or Ĥ b |H a)︁The notation Ĥ means that our statistical estimate of the estimated likelihood of Hexceeds the threshold needed for our decision to confirm it. This formulation carriessome caveats the justification for which is pragmatic and will be expounded upon inlater sections. The main one is the use of the term ‘drives’ in place of ‘causes’. Thenull hypothesis can be viewed as equivalent to strong Granger non-causality (as it willbe argued is necessary), but it does not mean that the signals A and B are independent:they may well be correlated to one another. Furthermore, we cannot realisticallyaim at statistically supporting strict Granger causality, i.e. strictly one-sided causalinteraction, since asymmetry in bidirectional interaction may be more likely in realworldobservations and is equally meaningful. By ‘driving’ we mean instead that thehistory of one time series element A is more useful to predicting the current state ofB than vice-versa, and not that the history of B is irrelevant to predicting A. In thelatter case we would specify ‘G-causes’ instead of ‘drives’ and for H 0 we would employnon-parametric independence tests of Granger non causality (GNC) which havealready been developed as in Su and White (2008) and Moneta et al. (2010). Note thatthe definition in (1) is different from that recently proposed in White and Lu (2010),which goes further than GNC testing to make the point that structural causality inferencemust also involve a further conditional independence test: Conditional Exogeneity(CE). In simple terms, CE tests whether the innovations process of the potential effectis conditionally independent of the cause (or, by practical consequence, whether theinnovations processes are uncorrelated). White and Lu argue that if both GNC and CEfail we ought not make any decision regarding causality, and combine the power ofboth tests in a principled manner such that the probability of false causal inference, ornon-decision, is controlled. The difference in this study is that the concurrent failureof GNC and CE is precisely the difficult situation requiring additional focus and it willbe argued that methods that can cope with this situation can also perform well for thecase of CE, although they require stronger assumptions. In effect, it is assumed thatreal-world signals feature a high degree of non-causal correlation, due to aliasing effectsas described in the following section, and that strong evidence to the contrary isrequired, i.e. that non-decision is equivalent to inference of non-causality. The precisemeaning of ’driving’ will also be made explicit in the description of Causal StructuralInformation, which is implicitly a proposed definition of H 0 . Also different in Definition(1) than in White and Lu is the accounting of potential error in causal directionassignment under a framework which forces the practitioner to make such a choice ifGNC is rejected.One of the difficulties of causality inference methodology is that it is difficult to ascertainwhat true causality in the real world (‘ground truth’) is for a sufficiently comprehensiveclass of problems (such that we can reliably gage error probabilities): hence theneed for extensive simulation. A clear means of validating a causal hypothesis wouldbe intervention Pearl (2000), i.e. modification of the presumed cause, but in instancessuch as historic and geological data this is not feasible. The basic approach will be toassume a non-informative probability distribution of the degree degree of mixing, or38