User Identifcation in Preprocessing WUM (cs-username)

In cs-username field , it only show “-” eventhough i used “forms authentication” in ASP.NET and IIS 5.1, Visual Studio 2005,Windows XP. I’m confused about it. In web usage mining’s user identification phase, we can idenfiy users by authenticated user or heuristics (IP,Agent,Refer) .
In my thesis,i ‘ll use authenticate user id . I use Forms authentication ,no-anonymous login . So,when user login , IIS web server logs the login user name in cs-username . But it doesn’t happen what i expected . cs-username field is “-” . Why?

IIS userautentication logs “DOMAIN\USER” as windows log in so, forms authentication, do i need to use ISAPI (Internet Server Application Programming Interface)
to get the login user name in cs-username ? Is it possible?

At first, i think
1. by using Login user, it is more easy to identify user.
2. i mine the web server logs of my own web site so “user login” identification is more suitable than IP,Agent,Referer Heuristics.

Disadvantages:
1. I’m in the stack . i can’t identify user because login user name doesnt’ appear in cs-username field (IIS Logs)
2. I’m not sure right now that i could do it with that cs-username. (in theoritically ,WUM useridentifcation can be made by authenticate user but, IIS & ASP.NET i ‘m not sure)
3. if i do it & if only it’s ok , i’m not sure my supervisors like that kind of user identification.
(Teachers may ask ” is it necessary to login ? why do i browse the books without registration”)
Another Approach:
I can use IP,Agent,Refer instead of Login for user identification .
There’r also problems .
1. I user one PC to login so IP address will be “localhost(127.0.0.1) . I can use agents as Firefox,Opera,IE.
2. Normally, users are visited web sites from many IP addresses so it is ok to use IP address .Some users use 1 proxy address so IP address will be the same .

Q::::can i make login username in cs-username field in IIS logs ????