The data comes from a blog on Discover[1], which is itself an analysis of the data from the General Social Survey[2].

The 16-75 is because the blog author groups the answers based on the score in the Wordsum test by the responder. People with higher scores are less keen on censoring others.

[1] http://blogs.discovermagazine.com/gnxp/2012/09/intelligence-...

[2] https://en.wikipedia.org/wiki/General_Social_Survey

The 16 and 75 were different people (the top 20% and bottom 12% of scorers on a vocabulary quiz that was included in the survey).