Turns out that Turnitin doesn't detect plagiarism. A recent pair of blog posts (here's one and here's the other) go to great pains to distinguish between similarity, which Turnitin does detect, and plagiarism, which it categorically doesn't. "As far back as 2013, Turnitin has been writing blog posts, speaking out, and generally finding any opportunity to clear up the question of whether our products detect plagiarism... The answer, however, is simple: Turnitin does not detect plagiarism."
The distinction Turnitin makes between similarity and plagiarism is that of evidence and judgement. The Similarity product merely "takes what a student submits and [compares] it to a massive database of content, including internet, academic, and student paper content, and [looks] for similarities." Plagiarism, on the other hand, is "intentionally representing someone else’s work as one’s own" (emphasis added). Similarity might be evidence of plagiarism, but strictly speaking, it's never more than that. A number of other factors—assignment requirements, genre, student skill level, to name a few—must also be weighed to determine whether student work shows that intention. Computers can count words. Humans can weigh the factors of a situation. Similarity can be detected by a computer, but the existence of plagiarism requires the judgement of a person.
It's good that Turnitin has this down in writing—because the marketing page for their Similarity product (which Populi integrates with) isn't shy about using the P-word. Web searches for "plagiarism-checker" invariably stick Turnitin's site at the top of the results (not least because Turnitin has bought, like, every other company in the market—and none of them ever hemmed or hawed about the phrase). For that matter, our own marketing materials blithely state that you can use Populi to "Help students stay responsible with... plagiarism checks for all kinds of assignments".
That's marketing for you. It takes shortcuts to get its point across. Turnitin has no desire to expunge phrases like plagiarism-checker from its site in favor of a nuanced discussion of similarities, evidence, and human deliberation. Marketing speaks in shorthand, and "plagiarism detection" is the shorthand for that whole discussion. We could talk about whether that phrase over-promises, whether it suggests that an endeavor of human judgment can be offloaded to a computer, but such subjects are amply covered by others. For now, the question is this: in light of Turnitin's profession that it "does not detect plagiarism," does Populi need to find a real plagiarism-checker to work with?
The short answer is No, not least because no such thing exists. Turnitin is a tool that efficiently checks work for similarities to other work; this is exactly what every other so-called "plagiarism-checker" does (or did before Turnitin bought them). We'll go on providing the integration for our customers, and, for better or worse, we'll continue to use terms like "plagiarism-checker" as shorthand for the service it provides. At the same time, this whole episode serves as a good reminder of what exactly we're doing here. Populi is college management software designed to serve the people who run schools, people who themselves are there to serve the other people at their schools. Ultimately, that comes down to providing a tool that helps you come to conclusions and make decisions on your own.