The Authoritative NSS Page

[Stock]

Oh, and the web site covers the data source here:

It clearly says:

...fails-to-deliver are not necessarily the result of short selling, and are not evidence of abusive short selling or “naked” short selling..."


How often is this data updated?
This data will be updated as soon as the SEC provides a new update. The last update included 1st half of July 2010 data and was added on 8/2/2010. As of July 2009, the SEC will begin releasing updated data on a two week delay instead of quarterly.

How accurate is this data?
The data is directly from the SEC which states "We cannot guarantee that the data will be posted by a particular date. We cannot guarantee the accuracy of the data." For the purposes of charting, the SEC data was adjusted by this site in the following way: Where the SEC did not track closing prices, they had inserted a "." into the data. These "."s were changed to "1"s.

How did the SEC track this data?
From the SEC: "The values of total fails-to-deliver shares represent the aggregate net balance of shares that failed to be delivered as of a particular settlement date if the balance is 10,000 shares or more. If the aggregate net balance of shares that failed to be delivered is less than 10,000 as of a particular settlement date, then no record will be present in the file for that date even if there are fails in that security. Fails to deliver on a given day are a cumulative number of all fails outstanding until that day, plus new fails that occur that day, less fails that settle that day. The figure is not a daily amount of fails, but a combined figure that includes both new fails on the reporting day as well as existing fails."

What is a Failure to Deliver?
A fail to deliver is recorded when a seller does not provide the shares of stock that were sold within the required 3 day settlement timeframe. From the SEC: "Fails-to-deliver can occur for a number of reasons on both long and short sales. Therefore, fails-to-deliver are not necessarily the result of short selling, and are not evidence of abusive short selling or “naked” short selling."

http://failurestodeliver.com/questions.aspx