restigious British journal <em>The Lancet</em> to appreciate the history and irony of “gold standard.”</span></div></div>
<p>Although <span class="math inline">\(Y \leftrightarrow X\)</span> systems provide no fulcrum by which to lever out the truth about the direction of influence, richer systems sometimes present an opportunity to resolve causal disputes with data. The choice of covariates via DAGs provides the necessary key.</p>
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