No proofreader is always 100% accurate. Many publishing houses use multiple proofreaders on the same job to ensure accuracy.
Proofreading is always performed on several levels.The measure of the quality of proofreading is in the percentage of errors missed and in the overall quality of the types of errors found within a given deadline.
In terms of quality and timeliness, I recommend to all of my clients that they ask themselves the following question:”Do I want it right, or do I want it right now?”
Strong proofreaders always work to strike a balance between the need for high quality and the need to meet deadlines. While proofreading the same material over and over would yield the highest quality possible, projects must be completed within a given amount of time, or nothing would ever appear in print — so an appropriate compromise between quality and timeliness must be struck.
Quality proofreading requires intense, continuous concentration for long periods of time (it is very hard work). As a rule, a proofreader should not proof for more than six hours in one day; after that, quality tends to suffer.