[john_flintstone]

What would figures for another European city tell us about London? Rates for programmers in Dublin are about 20% higher than London, but that says nothing about the London figures quoted here.

[MrQuincle]

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