Systematicity

[David Bowie]

In the thread "Different dialects, same error",
Dennis R. Preston wrote:

: There are sporadic existential 'it' occurrences among non-southern
: and non-AAVE speakers, but they are surely not systematic or common.

Define "systematic".

Seriously--this is a place where i'm running into a *lot* of trouble in some
back-burner research i'm doing. If a speaker does something allo the time in
a particular context, we can pretty clearly say it's systematic. If a
speaker does it *half* the time, we still can say it's systematic. But where
do we draw the line between systematic and non-systematic--one use in 10
opportunities to use a feature? One in 1,000?

If a speaker only uses a particular feature one time in 100,000 possible
opportunities to use it, wouldn't that still be systematic is, after 3
million opportunities the speaker had used the feature ~30 times? Or is that
below the threshhold of linguistic systematicity, since it'd be really
really hard to test for it, particularly for syntactic features?

David Bowie                                         http://pmpkn.net/lx
    Jeanne's Two Laws of Chocolate: If there is no chocolate in the
    house, there is too little; some must be purchased. If there is
    chocolate in the house, there is too much; it must be consumed.