Coupling efficiency is related to the amount of full-length sequence present following synthesis of your oligonucleotide. If each base is added to a 50 base oligonucleotide with the industry standard of 98.5% coupling efficiency, less than half of the final product would be full-length sequences. IDT exceeds this level of efficiency with coupling rates of 99.2% for standard oligonucleotides and 99.5% for Ultramer® Oligonucleotides. For the same 50 base oligo, this would result in greater than 67% and 78% full-length oligonucleotides, respectively. A higher percentage of full-length oligos will help to reduce concerns over the efficiency and reproducibility of your experiments. See the DECODED article, Getting Enough Full-Length Oligo?, for data and a longer discussion.