Several criticisms can be made of the study which reported that Medical school coaching does not help (01/09). The fundamental one is incorrect sampling methodology. This is best explained with an analogy. Let us say a comparison was made of the health status of those who see a doctor often with those who see a doctor rarely. The results of the study might indicate that the difference in the health status of the two groups is negligible or even that the health status of those who see doctors often is slightly worse. But, it is incorrect to then conclude that there is no benefit in seeing a doctor. This is because the people who see doctors often and those who don't are completely different populations and have different characteristics. For example, those who see doctors often may do so for several reasons (for example they may be sicker to start with).
Further, in the above example, it is inappropriate to generalise the effectiveness of seeing doctors. Most doctors are excellent whilst some may even make the illness worse! Similarly with UCAT and interview coaching there are some reputable organisations as well as some which are just plain dodgy. Lumping all of them together does not provide a true picture.