  By using effective field theory techniques for the standard model, we discuss
the issue of what $\mu$ scale is the appropriate one in the QCD corrections to
the large-$\mt$ electroweak contributions to $\Delta r$. This needs the
construction of an effective field theory below the top quark. We argue that,
while matching corrections do verify the simple prescription of taking $\mu
\simeq \mt$ in $\a(\m)$, logarithmic (i.e. $\sim \log \mt$) corrections do not,
and require the use of the running $\a(\mu)$ in the corresponding
renormalization group equation.
