Bill,

Your speculation on why they occur are all plausible.  I would just observe that your explanations require appeals to various language principles interacting with each other.  Such observations don't seem to match with a claim that our knowledge of language is strictly based on exposure to the language.

Of course, you may be right here. 

"I suspect we *do* encounter a lot of those mixed constructions -- we
just don't encounter them in writing. I know I've heard quite a few on
news broadcasts and the like. "

On the other hand, it is interesting to note that  Biber et al. in the Grammar of Spoken and Written English do not index "mixed construction" and all of the references to "by" makes no mention of them.  If they are frequent in the spoken language, this absence is strange.  

This is not the case elsewhere.  For example, Biber et al. mention prefaces.

(1) This woman, she's ninety.

It notes that prefaces occur in conversation and not in academic writing. (p. 964).

So, Biber et al. do note structures that occur only in the spoken language.

**
Let's clear up something about "my view" of such forms in developing writing.

Bill writes: ". . .  in your view, they're probably performance errors
and quite separate from what might count as evidence for linguistic
competence."

I'm interested in trying to understand why developing writers do what they do.  I take a developmental perspective on such constructions.  From the developing writer's perspective, I don't think these are performance errors.  Rather, they  represent something about such writer's competence.  I think teaching needs to begin with where the student is, so a perspective that tries to understand the writer's principles is much more useful pedagogically than a perspective that says this is what the writer should be doing.

One of the problems I have with systemic functional linguistics is that it really doesn't provide any insights into why developing writers do what they do. Halliday is quite clear his perspective of language is not about what a language user knows. Likewise, the belief that our knowledge of language is based solely on the language we have been exposed to doesn't offer much of an explanation except to speculate perhaps these structures are in the oral language and just haven't been noticed.

Bob Yates, University of Central Missouri  

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