Beware of the single rank x16 RAM kits sneaked in some laptop models! The 1Rx16 modules have significantly lower bandwidth compared to the 1Rx8 variants - NotebookCheck.net News
TECMIYO 8GB 2RX8 PC3L-12800S DDR3 1600MHZ SODIMM PC3 12800 1.5/1.35V CL11 204Pin 2RX8 Dual Rank Non-ECC Unbuffered Laptop RAM for Apple MacBook Pro iMac Mac mini: Amazon.co.uk: Computers & Accessories
How to Diagnose, Check, and Test for Bad Memory
For Samsung Chips 4GB 2GB 1GB Kits DDR2 DDR3 DDR3L Laptop RAM Memory SODIMM Lot | eBay