Dear All,
Tomorrow at Applied
Stats<http://projects.iq.harvard.edu/applied_stats> we will be
hearing from Tyler VanderWeele, Professor of Epidemiology at the Harvard School of Public
Health. Professor VanderWeele will be presenting on Surrogate Measures and Consistent
Surrogates. As per usual, the talk will be held in CGIS
K354<http://map.harvard.edu/?bld=04471&level=9> at 12 noon and lunch will be
served.
Abstract: Surrogates which allow one to predict the effect of the treatment on an outcome
from the effect of the treatment on the surrogate are of interest when it is difficult or
expensive to measure the primary outcome. There have, however, been several instances of
drugs that have been approved for use on the grounds of randomized trials using surrogate
outcomes, that have subsequently led to public health catastrophes, costing thousands of
lives. It is now clear that the use of surrogates can give rise to paradoxical situations
in which the effect of the treatment on the surrogate is positive, the surrogate and
outcome are strongly positively correlated, but the effect of the treatment on the outcome
is negative, a phenomenon sometimes referred to as the "surrogate paradox." New
results are given for consistent surrogates that extend the existing literature on
sufficient conditions that ensure the surrogate paradox is not manifest. Specifically, it
is shown that for the surrogate paradox to be manfiest it must be the case that either
there is (i) a direct effect of treatment on the outcome not through the surrogate and in
the opposite direction as that through the surrogate or (ii) confounding for the effect of
the surrogate on the outcome, or (iii) a lack of transitivity so that treatment does not
positively affect the surrogate for all the same individuals for which the surrogate
positively affects the outcome. The results are related to several common approaches and
measures for assessing surrogacy including the "proportion explained" and the
Prentice criteria, the "proportion mediated", meta-analytic approaches, and
principal strata effects. None of these measures or approaches entirely protect against
the surrogate paradox. An attempt is made to synthesize the existing approaches and
results into guidelines on avoiding the surrogate paradox and ensuring consistent
surrogates.
You can also find a link to the corresponding paper
here<http://projects.iq.harvard.edu/applied_stats/event/tyler-vanderweel…th>.
Tess
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Tess Wise
PhD Candidate
Harvard Department of Government
http://tesswise.com