Addendum number 100000921, released 5/9/2011, addresses this question. The Addendum for SSc4 applies to Option 3, Approach 1 where survey data is solicited from the entire population of regular building occupants. Previously, a project needed to achieve an 80% response rate to extrapolate the survey results to the entire building. New guidance from the Addendum lowers the survey response rate to 60%. The Addendum also offers a discounted extrapolation factor based on survey response rate levels between 30% and 60%. See the Addendum for additional information. A new USGBC calculator is available to assist in using the survey results to assess LEED compliance. Download the calculator from USGBC or from the LEEDuser Documentation Toolkit tab.