Without even looking at potential issues with the source data, it is also the case that most programmers of any calibre work across multiple languages. Also, languages which are 'older' and reaching obscurity (ie. fallen out of favour) are going to be known by older programmers who, if they are still in their career, probably command higher salaries only as a factor of age and experience. Finally, salary before tax is not very representative of income or social circumstance, and people in our industry often opt for time flexibility or remote work in preference to higher salaries on purpose (for example, after having children). All in all, not useful.