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So, we
know how the tax change is supposed to work. We know the output of the payroll
system is a correct paycheck for the correct person. We begin by identifying what
we think are the appropriate programs to update with our mandated tax change; we
review and analyze what the inputs, processing, and outputs are of each program;
we identify and analyze the interfaces with each identified program, other programs
in the payroll system, and any external programs or processing that might
interact or be affected by the change; we make the change; and we test it. And
those of you who have done this know it blows up because it is impossible to do
all of the aforementioned steps correctly with such a diffi cult, poorly documented
system. The point is, we tried to decompose the existing system into its parts, and
then tried to figure out how those parts worked together. Then, we defined how the
change would affect the old way of calculating payroll. This is reverse engineering.
We figured out how to make something work based on how it worked before.


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