This week Kobo became the latest organisation to attempt to use its data to estimate the overall size of the global eBook market. Its inaugural Book Report measures the value of the current global market for eBooks at $14.5 billion and predicts that this will grow to $22 billion by 2017.

Kobo doesn't disclose how it arrived at this number but it is very possible that it has made its predictions by extrapolating them upwards from its own sales figures. This gives Kobo's estimation an edge over other predictions made using data from second-hand sources or drawn from consumer panels. It does, however, rest on the assumption that Kobo's data set is large enough to support robust predictions. And on the assumption that Kobo's performance is a direct indicator of what is going on in the general market.

According to the small-print in Kobo's announcement, its Book Report was based on data drawn from 21 million users across an unspecified number of the 190 countries in which it has an active presence. At first glance this seems impressive, but as we discovered from our own research on mobile reading habits earlier this year, Kobo is still very much a minority player in the eBook space, having only 9% of the UK's digital reading market and even less share in the US. This means that even if we assume that a reader of eBooks on Kobo behaves in the same way that a Kindle or iBooks reader, Kobo still has no visibility into the remaining 90% of the eBook market.

It's also difficult to determine the accuracy of Kobo's assertion that booksellers sold $14.5 billion worth of eBooks in 2014, as no two companies or organisations use the same data to measure the market. For example, analyst firm Strategy Analytics released a report in November 2014 that undershot Kobo's predictions. It valued the global eBook market in 2013 at $7 billion -  and predicted it would grow to $16.7 billion by 2020.

Meanwhile PriceWaterHouseCooper's Media & Entertainment Outlook, released in June 2014, fell somewhere in between Kobo and Strategy Analytics. It measured the size of the consumer eBook market in 2014 at around $11 billion and predicted it would grow to $17 billion by 2017. All three of these estimates are, however, much more conservative than those put forward by other analysts. For example, writing in January 2013, Forrester Research was predicting that the European eBook market alone would be worth $19 billion by 2017.

These predictions all have one thing in common: an expectation that there is still a lot of growth left in the global eBook market. Where this future growth will come from is a larger question As we reported earlier in the year, while overall eBook sales volumes grew in the US over 2013, actual revenue from these eBooks declined. If eBook sales in the US continue this trend in 2015, selling more units at lower prices, the real growth in the eBook market will come from elsewhere.

In 2013 Forrester was convinced this growth would come from Western Europe, where eBook penetration remains low everywhere except the UK and Germany. Strategy Analytics' more recent 2014 report, however, asserted that increased demand for eBooks in China would drive demand. Wherever this growth comes from, to meet the expectations of analysts, booksellers and publishers it will need to be significant to meet the great expectations of all involved.