Registration: Username and password or email and password

During a registration process, what would the reasons be for using either username/password or email/password combinations to register the user?

• Is it a case of identification once in the system (it may be preferable to refer to a user by their username instead of their email)


• Simplicity - using email means the user doesn't have to create two
  
  memorable items?

What other reasons are there for choosing either option? Is there a relationship between the types of service?

Answer

For the same number of fields (2) you get more useful information about the user. Both username and password are content free, but an email address is not.

In addition, my experience has been that using email address rather than a username bypasses people's privacy filter. Many people avoid giving out their email address, or use a throwaway address when they are asked specifically for it in a form. But when it is their identity, they are more likely to use the address that they think of as 'them', which means not a throwaway.

Finally, in forum/comment environments you also need to ask for a Display Name; phrased in this way, people are more likely to use their real name than an alias. It serves the same purpose as the original username, but again in my experience a far larger percentage of users provide their real name, but since they don't have to provide their real name it does not turn away or offend those who value privacy like 'First Name', 'Last Name' fields do.