The fundamental flaw in the UWS study has been exposed elsewhere, but it is suffice to say that the universities and ACER have a vested interest in promoting the myth that coaching does not help (Is coaching crucial? MedObs 5/12/08, p15). Whatever means is used to select students into high demand courses such as medicine, parents will be keen to ensure that their children have the best chance of getting in with coaching, whether for year 12, UCAT or the GAMSAT (test for graduate entry).
It is true that it is difficult to precisely quantify the benefits of any educational service. For example, it is impossible to validate the claims of universities that obtaining a degree significantly increases the lifetime earnings of graduates, because students will have done well in the earnings stakes due to their intellectual and emotional intelligence, regardless of whether they had obtained the degree.
Ten years ago, ACER was adamant that even familiarity with the test was unnecessary, so they did not release any information on the test. Now they are providing practice exams. Within a decade, people in Australia will come to accept that aptitude tests can be coached for, as has happened in the USA.
In fact, much of the opposition to high stakes testing in schools is based on the premise that teachers will 'Teach to the Test' (i.e. teaching for the tests is so effective that teachers will engage in this, at the exclusion of teaching other things).