I was wandering around the Internet the other day (since I have nothing else do while my brain organizes my next plot) and I came upon what must be the third mistake of professional writing. I saw at least three sites out there asking for entire chapters of an e-book as a sort of "test" for writers, promising that if these writers meet their standards, the writers will each get a contract to write an e-book. Paid and everything.
This sounded odd to me, as a professional writer. So, I decided to enlist the aid of a couple of online writing pals to investigate one of them. We each signed up and received a description of the potential e-book and were told which chapter would be our "sample" chapter. Interesting fact: all three of us were assigned different chapters in the same e-book. If this book had 14 chapters (which it appeared to), and the site managed to get 14 people to write "sample" chapters, then they get an entire e-book without ever having to "hire" any of the writers. Sneaky, sneaky.
The lesson here: a "sample" should not be an entire chapter or article. A "sample" would usually consist of a very short work, just enough to show your abilities as a writer (and a researcher, as most writing requires some research). Don't put your faith in false promises and hand over more of your work than necessary. Best case: use something already published as a writing "sample." At least you've already been paid for it.