The effect of omitting <i>z</i> from the health outcome equation.

Abstract

<p>In theory, the total effect of <i>z</i> on health outcome, H, is <i>δ</i> + <i>θd</i>. In practice, using <i>z</i> as an instrumental variable causes bias in the estimated effect of the 25(OH)D score, <i>D</i>, on the health outcome, because the direct effect of <i>z</i> on the health outcome is incorrectly captured as being mediated by <i>D</i>.</p