Overall, this approach entails a high level of external validity and applicability but often only a low level of causality.
66 There is also emerging evidence from several studies that lead could be implicated in ADHD even at low levels, but causality cannot be assumed from the evidence to date.
Intuitively, we would expect Granger causality at low frequencies to be preserved by filtering and down-sampling.
The chi-square results also suggested low potential for reverse causality.
Future longitudinal studies are needed to confirm the causality between low SES and MetS.
It is difficult to prove causality between low testosterone and COPD, since multiple mechanisms are likely to act simultaneously.
Clinical studies suggest a possible inverse IGF-liver function association, although reverse causality, with low IGF-I levels being a consequence of liver damage, remains a possibility (Wu et al, 1988; Buzzelli et al, 1993).
In fact, a variant of intron 2 of HLA-A was reported as causality of low expression of the HLA gene, furthermore, variants in intron 2 of HLA-A, introns 2 and 4 of HLA-B cause null expression by cryptic splicing activation [ 22- 24].
As Brock (1991) illustrates, linear Granger-causality tests can have low power uncovering nonlinear causal relations, which could also exist among variables.
In particular, the experiment ensured that social rank was the only factor being changed, providing strong evidence that the chain of causality runs from low social status, through a disrupted immune system to worse health, and not the other way around.
