Michael Geist sez,
The NPD Group response contains math errors for both non-P2P users (the total should be $192 not $191) and P2P users (the total should be $268 not $267), though perhaps this is due to rounding errors from the original data. More important, however, is a bigger math error in the chart as NPD Group significantly understates the difference between P2P and non-P2P users. In arriving at the grand total, it adds all the categories (physical CDs, paid downloads, subscription fees, merchandise, and concert tickets) plus the sub-total. In other words, it double counts the physical CDs, paid downloads, and subscription fees. The actual grand total of the five categories of spending is $206 per year for P2P users and only $138 for non-P2P users for a difference of 49.3 percent. There is obvious irony in NPD Group talking about the need for a licence to publish data only to get its math wrong, yet the real significance is that few would credibly argue that a nearly 50% increase in spending by P2P users can be simply chalked up to unsupported claims that P2P usage had no impact on consumer purchasing behaviour.
NPD Group Fails Basic Math: Data Shows P2P Users Spend Nearly 50% More on Music Than Non-P2P Users