National Association of Information Destruction (NAID) in Australia recently completed a very basic, but profound study. The Secondhand Hard Drive Study, completed in January 2014 and published February 19th, 2014, showed that a third (15 of 52) disk drives acquired from various random sources were still containing significant confidential personal information. Of the fifteen containing confidential information, roughly a half were acquired from individuals. However an equal number came from attorney firms, medical entities, non-profits and government entities. The drives came from a variety of normal sources of used computer equipment such as ebay. The data extraction did not require a significant amount of technical expertise to retrieve. Bob Johnson, the President of NAID said "Had the data been properly erased, it could not have been found." However, keep in mind, to properly remove data, it requires more than just pressing the delete button.
You see the complete study here. http://www.naidonline.org/naus/en/consumer/news/5163.pdf