The most important measures against plagiarism are those that create preventative effects, that is, which prevent plagiarism altogether.
URKUND has a very strong preventative effect, with a number of factors contributing. Implementing a system like URKUND sends a clear signal that plagiarism is unacceptable and that the resources to address the problem are in place. Having students run their papers through the system also helps to reinforce its preventative effect.
The ability to check all submitted documents creates a preventative effect in that students know there is no chance of their papers not being checked. Tools that involve checks based on random sampling or that are only used when the instructor suspects plagiarism do not create this preventative effect. In these situations, students know that hiding typical plagiarism indicators will increase the likelihood of their document not being checked.
URKUND checks material from all relevant sources. This means that there are no 'safe' sources a plagiarist can use, which further reinforces the preventative effect. Tools that only check material from the Internet do not have this effect, since students understand that they can copy from other students' work or non-digital articles, books, and similar – and thereby run less risk of being discovered.