220 million units is still a fairly solid figure, but this research is basically saying that people are switching to larger smartphones and keeping their tablets longer. (goodereader.com)
I did some detailed guessing in a prior post, but would rather see more solid figures than my own back of the envelope attempt. (dotearth.blogs.nytimes.com)
Fuel sold at retail seems to me to be the most solid figure here. (realclimate.org)