Originally Posted by veridicus
3 is the most common to try to achieve. It's a very practical level. 4 is difficult. 5 is generally considered an ideal that no one even tries to achieve. There's a big difference between academic ideals and practicality.
There's an old joke about a job interview question. A typical interview question for a software developer used to be "What is 3rd form normalization?". Of course the answer is, "It's between 2nd and 4th form normalization."
Yeah, most places I have read and other people in the industry say 3 is the most common.