To be more accurate, a survey of 1200 ereader owners showed that 40% of respondents siad they read more now that they have an ereader.
A study of 1,200 ereader owners by Marketing and Research Resources Inc. found that 40% said they now read more than they did with print books. Of those surveyed, 58% said they read about the same as before while 2% said they read less than before. And 55% of the respondents in the May study, paid for by ereader maker Sony Corp., thought they'd use the device to read even more books in the future. The study looked at owners of three devices: Amazon.com Inc.'s Kindle, Apple Inc.'s iPad and the Sony Reader.
I'm not so certain that you can get reliable data from this type of poll. I suspect that the increase in reading is actually a lot greater than 40%. I also suspect that the respondents are also reading more on their PDAs and smartphones but don't realize it.
But in order to measure the real numbers you'd need to follow people around with a clipboard (or install tracking software on their handheld). And that takes a lot more time and money.