SPSS Basic Sskills Tutorial: Data Entry
http://my.ilstu.edu/~mshesso/SPSS/data_entry.html

Harvard/MIT Guide to SPSS
www.hmdc.harvard.edu/projects/SPSS_Tutorial/spsstut.shtml

e-Source: Chapter 4 'Concepts in Sample Surveys' by Sarah M. Nusser and Michael D. Larsen
www.esourceresearch.org