What yield and density did the cartridges achieve in testing?

The yield is the amount of pages the cartridge produces before running out of toner. Make sure that the testing is performed at 5% page coverage. This is the industry standard. You can ask if they used a toner head, this is the best way to achieve 5% page coverage. Be careful with companies that just give you the OEM yield, they may not have actually done any testing.

The density measures how dark the text is. The density is usually between 1.3 and 1.6. Ask how many cartridges they test to achieve their numbers. Testing one cartridge does not give an accurate measurement. Also, ask how often they test their toner/drum combinations. Suppliers change toner qualities and continued testing is important to ensure quality.