How Effective are Plagiarism Detection Services?

According to Debora Weber-Wulff at the University of Applied Sciences in Berlin, plagiarism detection software is not making the cut. She recently tested 26 plagiarism detection services, finding that the most effective services detected only 70% of plagiarism.

The top 5 systems were deemed "partially useful" and received a grade of C-:

1: PlagAware
2: Turnitin
3: Ephorus
4: PlagScan
5: Urkund

At the bottom of the list were the following "useless systems for education":

22: PlagiarismSearch
23: PlagiarismChecker
24: Grammarly
25: PercentDupe
26: ArticleChecker

The University of Applied Sciences in Berlin makes the following recommendation: "The focus should be on teaching students about plagiarism and how to avoid it instead of investing time in using software."

The test overview and summary can be found in English here:
http://plagiat.htw-berlin.de/software-en/2010-2/

Debora also writes a blog dedicated to plagiarism:
http://copy-shake-paste.blogspot.com/