Reliability for summation index from binary variables

#1
Hello all.

This might be simple to some of you but I'm stuck with it at the moment. Hence this call for help.

I'm trying to create a sum index call "social media use" from three binary variables: use social networking sites, use blogs and use video hosting sites (all coded 0=No and 1=Yes).

Would be very grateful if you could show me how to assess the reliability of this index. Many thanks in anticipation.

An