The other day I was contacted about advertising on learningandteaching.info. That is not unusual, and I always turn it down--the site is not a commercial venture, and the expenses are trivial at the moment. However, on this occasion the proposal was to promote free plagiarism checking services, and suggested links from my page on plagiarism.
There is indeed a good case for such services. Probably most students' institutions now use plagiarism checking software, principally Turnitin, and require its use at every point of assessment submission. That's fine for them, but not such a good deal for the student, who is frequently not allowed to use the service to get formative feedback on their work--there are various reasons for this, but they chiefly focus on security issues. So, although the sheer size of the checking database is important, a lighter-weight service tailored to the student's needs can be very useful as a way of ensuring that they do not stumble unintentionally into plagiarism--after all the consensus is that much plagiarism is the consequence of ignorance of academic conventions about citation and originality, rather than deliberate cheating. (See links from the page cited above.)
So I entertained the idea of linking to some of these services, as a service of my own rather than via their advertisement, and I went to the urls provided, to find both of them point to Grammarly, which sells itself principally as a grammar checker and whose plagiarism tools are very limited in the free version.
The other plagiarism checker is apparently free-standing, but touts itself as a means of ensuring that web content is original, because that helps with search engine optimisation. That is not quite the same as assuring academic integrity, and of course it works only from public web pages. It also has a (proper) grammatical error in its submission box; "watch the article re-writer do it's thing". I suppose it may be some limited use, but if you can access a proper service, do so.
Grammarly gave no useful information at all about what content was not unique; as I had rather expected, payment would be required for any more detail. It emphasises its use as a grammar-checker. Moreover, the list of "grammatical rules" which flashed past was spurious. It included "split infinitives" and "prepositions at the end of sentences" and use of the passive voice among many other items which concern obsessive pedants and no-one else. It gave a "score" of 44% to a section of a forthcoming book which has already been proof-read and edited by real live people. Of course there was no recognition of the register or style of the piece (that is available on paid versions), nor of differences between US and UK usage and spelling.
It has been reviewed to similar effect and in more detail here, and here.
But it's not merely the technical incompetence of the service which concerns me. More pernicious is its impact; if I were a less-than-confident student getting a report like that I would feel incompetent and highly discouraged. I am not in favour of "going easy" on students lest they be discouraged--they do need accurate, stimulating and usually uncompromising feedback. They do not need to be treated as "fragile learners" to use Kathryn Ecclestone's term*.
But services such as these are nit-pickers on steroids; they mistake adherence to aribtrary "rules" for good practice in writing, and generate innumerable Type I errors as testimony to their rigour. They are con-tricks, and should be avoided, which is why I have not linked to them.
* Ecclestone, K. (2010) Transforming assessment in lifelong learning, Buckingham: Open University Press