Usability for Nerds/Software/Default values

Whenever the user has to fill out a text field in a form or make a selection from a list, the system may preset it to a default value so that the user doesn't have to do anything if this value is acceptable. There are several ways in which the software can determine a default value:

1. Always set it to the same value
2. Set it to the same value as the current user entered last time
3. Set it to the value used most often
4. Try to guess the value from something else the user has entered

Method 3 and 4 can be quite confusing because the default value is possibly not always the same and it may change in a way that is not transparent to the user.