Criterion 3: Yield. The ideal or theoretical yield of a chemical reaction would be 100%. According to Vogel, yields around 100% are called quantitative; yields about 90% are excellent; 80%, very good; 70%, good; 50%, fair; and yields below about 40% are considered poor. Purification steps such as distillation or recrystallization always lower the yield, and the reported yield usually refers to the yield of the final purified product...
[snip] The objective of the development chemist is to reach yields of 80% or above. Yields in the range of 70% and below are often considered unfavorable in the pharmaceutical business. The lower the yield, the larger are the number and relative amounts of side products, which can carry over to subsequent steps and potentially remain undetected. Since questions can be raised about the destiny of the unaccounted materials during regulatory inspections, particular considerations are given to the monitoring and fate of genotoxic impurities, and avoidance of genotoxic impurities has become an increasingly important synthetic design consideration. Finally, for low-yielding steps, the cleaning operations can become complex and more involved.To this novice process chemist, the reminder of the further problems with low yields (undetected side products, problems with cleanouts) are important. And now you have it -- when someone says "and this oxidation was carried out with an excellent yield of 83%...", you can remind them that, ahem, Arthur Vogel says that your yield was "very good."*
*Don't do that, it's not nice to be mean.
1. Dach, R.; Song, J.J.; Roschangar, F.; Samstag, W.; Senanayake, C.H. "The Eight Criteria Defining a Good Chemical Manufacturing Process." Org. Process Res. Dev. ASAP. dx.doi.org/10.1021/op300144g