Take enough sample of behavior. Other things equal, the more items that you have on a test, the more reliable that test will be. This seems intuitive right. While it is important to make a test long enough to achieve satisfactory reliability, it should not be made so long that the candidates become so bored or tired that the behavior that they exhibit become unrepresentative of their ability. At the same time, it appropriate. The usual argument for shortening a test is that it is not practical.