Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
purpose a given comma is serving-is it separating one main element
from the next, separating subunits within a main element or separating
grammatical units within an element, such as a clause from a
concluding phrase? Divisions that are obvious when sentences are
short may become fuzzy with longer constructions, and the reader
may sometimes have to pause or backtrack to stay on top. Using
the stronger separator of the semicolon makes the divisions stand
out better. For example:
Recruiting participants for the study included screening all subjects to
ensure that they had no contraindicating medical conditions;
distributing an information sheet describing the protocol to those
subjects meeting our inclusion criteria; arranging for eligible subjects
to participate in post-treatment discussion groups with the goal of
providing feedback to the researchers; and designating a group leader
to attend these sessions and coordinate the discussions.
All this sentence is ultimately doing is listing four activities, and
if its elements were shorter, commas as separators would work
just fine (Recruiting participants included screening subjects,
handing out information, setting up discussion groups, and
designating a group leader). However, because there is so much
going on, the divisions between elements are easier to see if semicolons
are used instead. Just what constitutes "long" elements will
be a matter of judgment; there isn't some magic number that decrees
when semicolons become necessary. The only guideline that can be
given is to use them if you have reason to think they would make
a sentence easier to read.
Finally, even if adjoining elements do not contain any internal
commas and are not that long, you may sometimes decide to separate
them with semicolons if each one deals with a distinct subject, in
order to make this distinctiveness stand out more clearly. (See page
55 for the grammatical definition of "subject.")
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Not so long ago, authors turned in manuscripts composed on
typewriters, and their entire text would have to be rekeyed by a
typesetter. These days, they usually provide the publisher with their
work on a computer disk; any editorial changes are made directly on