  The derivation of scattering equations connecting the amplitudes obtained
from diagrammatic expansions is of interest in many branches of physics. One
method for deriving such equations is the classification-of-diagrams technique
of Taylor. However, as we shall explain in this paper, there are certain points
of Taylor's method which require clarification. Firstly, it is not clear
whether Taylor's original method is equivalent to the simpler
classification-of-diagrams scheme used by Thomas, Rinat, Afnan and Blankleider
(TRAB). Secondly, when the Taylor method is applied to certain problems in a
time-dependent perturbation theory it leads to the over-counting of some
diagrams. This paper first restates Taylor's method, in the process uncovering
reasons why certain diagrams might be double-counted in the Taylor method. It
then explores how far Taylor's method is equivalent to the simpler TRAB method.
Finally, it examines precisely why the double-counting occurs in Taylor's
method, and derives corrections which compensate for this double-counting.
