Taking a test once gives us statistical information for validating individual questions. However, what is *really* useful is when a person takes two tests, preferably on two consecutive days. The questions are chosen from a large pool at random, so the questions in the second test will most probably be different from the first one. This allows us to correlate the two results, thereby validating the whole test.
We have been offering 10 test credits to people who have taken two tests (see the site for what test credits are and their worth). The credits will be usable for any tests. Caveat: at the moment there are no tests available for use in anger.
Hope this helps.