One of the big puzzles in trying to understand the tablet market is data. Apple is pretty much the only large manufacturer giving unit sales, and not only are Samsung and Amazon saying nothing, but there are hundred of Chinese companies making Android tablets, and few if any say anything publicly. In the phone business the story is rather easier, since Google says roughly how many Android devices are being activated outside China, and give a screen size breakdown, and the Chinese mobile operators have a good sense of smartphone sales inside China. But in tablets there is no-one with an overall view of sales.

This means that analysts have to fall back on component makers. Though there are hundreds of companies making tablets, they almost all use more or less off-the-shelf chipsets from a handful of companies. These companies have a good sense of the overall number of tablet chips that are being sold into the manufacturers. So too do the screen markers - they have a good sense how many 7” panels are being sold. So you take an estimate of tablet chip sales and an estimate of tablet screen sales (and take into account people using ‘phone’ chips in tablets) and you estimate tablet sales, both inside and outside China.

The obvious problem with this is that these devices are being used in very different ways. It seems clear that most of the huge numbers of sub-$150 Android tablets now being sold do not have anything like the web or app usage that is seen on an iPad, Nexus or Galaxy Tab, and that many are mainly used as substitutes for TV sets, with maybe some gaming on the side.

The deeper issue, though, is that estimating tablet sales in this way is a little like trying to estimate global car sales by working out how many internal combustion engines are being made, and how many tyres, but not adjusting for motorbikes, cranes, outdoor generators or 18 wheelers. Lots of ‘tablet' chips and ‘tablet' screens do not actually end up in tablets.

Consider this device, one of thousands of similar products on offer on Alibaba - an in-car video player running Android, complete (if the screenshot is to be believed) with the phone app. This probably doesn’t activate with Google, but it certainly looks like a ‘tablet’ to LG or Rockchip.