We’ve all gotten messages from our printers telling us to replace the ink cartridge, but it often seems like there's ink still inside.  PC World decided to do some real lab testing on this issue and the results confirm what you may have suspected: Many manufacturer-branded and third-party vendor cartridges leave a startling amount of ink unused even after the printer reports the cartridge is empty. In fact, some inkjet printers force users to replace black ink cartridges when the cartridge is nearly half full. Mark Sullivan and Tony Leung talk you through our results and testing procedures. And after watching the video, check out our full results and stats.