1 Introduction  
2 Research Background  
2.1 Undercoverage and Nonresponse in Web Surveys  
2.2 Internet Use and Health  
2.3 Selection Mechanisms and Weighting  
3 Survey Agencies and Recruitment Strategies  
3.1 Questionnaire and Pretest  
3.2 Dependent Variables  
4 Methods  
4.1 Methods for Comparisons  
4.2 Weighting  
5 Results  
5.1 Differences Between Unweighted Estimates  
5.2 Effects of Weighting  
6 Summary  
7 Discussion and Conclusion