linux - Why can't I delete a file where I have group write permissions on?

I have a file with the following permissions:

root:data, and chmod set to 775.

My normal user, let's call him boby, is in the data group.

Why can't I delete the file with the user boby?

 rwxrwxr-x 18 root data 4096 2011-12-30 22:02 storage
 my user is in the group data but can't write into storage