No reason to interpolate, for 2020 you would have 28.8ZB*sqrt(2)=40.7ZB, which is exactly the figure they are giving...
An IDC study sponsored by storage giant EMC tells us that, surprise, we need to buy more storage because the digital universe will reach 40 zettabytes by 2020. That's 40 trillion gigabytes and is estimated to be all the data created, replicated and consumed in that year. It's the result of the amount of digital information in …
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Sent to me anonymously and posted here as the quickest way to get the comment known:-
In your story, "Are you ready for the 40-zettabyte year?", you write, "The amount for 2020 would be 43.2ZB by interpolation." The increase is doubling every 2 years so you cannot use linear interpolation.
Half way between 2019 and 2021 the data will be closer to the 2019 amount than the 2021 amount. So 40 zettabytes looks reasonable for a continued doubling every two years.
Is he right? Am I innumerate?