1. Electronic mail was not better than postal mail for surveying residents and faculty;*Akl;Journal of Clinical Epidemiology,2005
2. American Association for Public Opinion Research (2006). Standard Definitions: Final Dispositions of Case Codes and Outcome Rates for Surveys, Ann Arbor, Michigan: Author, http://www.aapor.org/pdfs/standarddefs_4.pdf.
3. An experimental study of language intensity and response rate in E-mail surveys;Andersen;Communication Reports,2004
4. Monetary incentives in mail surveys;Armstrong;Public Opinion Quarterly,1975
5. Tracking the progress of e-mail vs. snail mail;*Bachmann;Marketing Research,1996