  Semileptonic and purely leptonic decays of B meson to $\tau$, such as $B\to
D^{(\ast)}\tau\nu_{\tau}$ and $B\to\tau\nu_\tau$ are studied. Recognizing that
there already were some weak hints of possible deviations from the SM in the
measurements of $\mathcal{B}(B\to\tau\nu_\tau)$ by \Babar~and Belle and the
fact that detection of the $\tau$ also occurs in the measurements of $B\to
D^{(\ast)}\tau\nu_{\tau}$, we stress the importance of joint studies of these
processes, whenever possible. For this purpose, as an illustration, we
introduce the observable, $\mathcal{R}(D^{(*)})/\mathcal{B}(B\to\tau\nu_\tau)$
where, for one thing, the unknown systematics due to $\tau$ identification are
expected to largely cancel. We show that all measurements of this observable
are consistent with the existing data, within somewhat largish experimental
errors, with the predictions of the SM. We stress that precise experimental
measurement and comparison with theory of the branching ratio for $B \to \tau
\nu_\tau$ is extremely important for a reliable search of new physics.
Furthermore, in view of the anticipated improved precision in experiments in
the next few years, in addition to $\mathcal{R}(D^{(*)})$, host of other ratios
analogous to $\mathcal{R}(D^{(*)})/\mathcal{B}(B \to \tau \nu_{\tau})$ in the
SM are suggested for lattice calculations as well, so that for more stringent
tests of the SM, correlations in lattice calculations can be properly taken
into account to enhance precision.
