Waterfall isn't really a phase. It's the entire process.

Something like, "describe your methodology?"

Followup:

What works?
What doesn't work?


>Ok, I get what you're saying.
>
>How would you put this into a survey? Would you say, "In your opinion, what phase of the the lifecycle most hinders project completion and why?"
>
>I'd ultimatly like to be able to quantify this.
>
>Thanks