Yesterday the Publisher’s Association put out the press release on its 2015 figures  with the headline ‘Strong Year for UK publishing industry.’ Total revenues increased by 1.3% (or about £100m) to £4.4bn. With inflation around zero last year, this is a real terms increase. The release highlighted the increase in physical book sales (and particular non-fiction – but I don’t know if this category includes the depressing proliferation of adult colouring books); and the first recorded decrease in digital sales.

You have to buy the statistics, so I’ve only done a few sums on the figures in the press release. This is not helped by the fact that the author of the release was a bit hazy about the difference between millions and billions, so there was a bit of guesswork involved. Revenues from physical book sales were up 0.4% to £2.76bn (an increase of around £12m). Revenues from digital sales were down 1.6% to £554m. Interestingly, then, physical sales continue to be much larger in scale.

And what about the gap? There is a line stating: “2015 was a great year for learned journals sales and demonstrates the strength of academic publishers in driving new innovative business models that contribute towards maintaining the UK’s position as a hub of global research excellence.” Piecing together the figures, it looks like a 5% (or approx £50m) increase to £1.1bn in sales of academic journals.

Export sales were down slightly – the rest of Europe accounts for 35% of the UK industry’s sales. As far as I can tell, the export figures are included in the above three categories (physical, digital, academic journals).

So all in all, yes a good year, pleasing for those of us devoted to physical books. But most pleasing of all to the publishers of academic journals (and not so good for taxpayers and students who fund the library purchases).

I spotted some tweets this morning about increases in the sales of physical media, including books.

These developments seem to overturn the narrative of ever-greater cannibalisation by online consumption.

But that narrative assumes physical and digital versions are always substitutes for each other, when there is no a priori reason to think that substitutability is boundless. The same substitution assumption was made about telephones when they first spread, but the empirical evidence seems to be that telephone and face-to-face contact might have become complements for each other rather than substitutes. The easier it became to arrange to meet someone in person, the more often it happened.

This is unlikely to be true for all physical media. While it is easy to see how reading part of a book online, or an article about a book, could start to increase physical sales, or how listening to music online can stimulate concert-going and maybe CD buying (even LPs!), it is less obviously true about newspapers, say. I don’t want to be all Pollyanna about this – and Anita Elberse’s [amazon_link id=”0805094334″ target=”_blank” ]Blockbusters[/amazon_link] for one provides some of the gloomier evidence about the effect of the switch to digital on most content suppliers even if the aggregate figures look healthy.

But the moral is, at least be aware of what you assume about substitutes and complements when predicting the future impact of digital; if you haven’t thought about it, you’ve still made an assumption. It might be wrong.

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