What's the overlap between Sufi Muslims and Sunni and Shiite Muslims?

A Pew Poll has given the percentages of Sunnis, Shiites, and those who identify as "just a Muslim" throughout the world. Each of these is considered mutually exclusive by the pollster. Then it says: >For some Muslims, another layer of identity comes from membership in a Sufi order. ... These orders can fall within either Sunni or Shia Islam. It then gives the percentages of Sufis in various countries, as it had for Sunnis and Shiites. But it doesn't really give an idea of the overlap between them. Have any polls or studies said how many Sufi Muslims also adhere to Sunnism or Shiism?