  The measurement of the branching fraction of a heavy quarkonium decaying into
light hadronic final state at $e^+e^-$ colliders is revisited. In $e^+e^-$
annihilation experiments, background contributions from the continuum amplitude
and its interference with the resonance amplitude are irreducible. These
effects become more and more significant as the precision of experimental
measurements improves. While the former can be easily subtracted with data
taken off the resonance peak, the latter depends on the relative size and phase
between the resonance and continuum amplitudes. Two ratios are defined to
estimate the size of these effects, $r_{R}^{f}$ for the ratio of the
contribution of the interference term to the resonance term and $r_{c}^{f}$ for
that to the continuum term. We find that $r_{R}^{f}$ could be as large as a few
percent for narrow resonances, and both $r_{R}^{f}$ and $r_{c}^{f}$ could be
large for broad resonances. This indicates that the interference effect is
crucial for the measurements of the branching fractions aiming at the percent
level or better precision and needs to be measured or estimated properly.
