In order to help compare processes, a measure called the E factor can be used. This was devised by Professor Roger Sheldon link to and compares the proportions of waste material with desired products.

In this context, waste will include any reaction products that do not have any further use, and also reagents and solvents used during the course of manufacture that are not re-used or recycled.

E-factors vary enormously between sectors of the chemical industry, but a "good" E-Factor would typically be around 0.1 - this would mean every 10kg of desired product produces 1kg of waste and by-product. At the other extreme, in pharmaceutical manufacturing when a high-purity product is essential, the E-factor can be as high as 100, meaning every 1kg of product produces 100kg of waste.