Of course the data tells you something. - http://en.wikipedia.org/wiki/Non-response_bias http://en.wikipedia.org/wiki/Non-response_bias - http://en.wikipedia.o

Of course the data tells you something. - http://en.wikipedia.org/wiki/Non-response_bias http://en.wikipedia.org/wiki/Non-response_bias - http://en.wikipedia.org/wiki/Social_desirability_bias http://en.wikipedia.org/wiki/Social_desirability_bias (quote: Personal income and earnings, often inflated when low and deflated when high) Something to cope with it: http://en.wikipedia.org/wiki/Heckman_correction http://en.wikipedia.org/wiki/Heckman_correction