Blood alcohol tests assume the individual being tested is average in various ways. For example, on average the ratio of BAC to breath alcohol content (the "partition ratio") is 2100 to 1. However, the actual ratio in any given individual can vary from 1700:1 to 2400:1, or even more widely. Thus a person with a true blood alcohol level of .08 but a partition ratio of 1700:1 would have a .10 reading on a Breathalyzer.

A similar assumption is made in urinalysis. When urine is analyzed for alcohol, the assumption is that there are 1.3 parts of alcohol in the urine for every 1 part in the blood, even though the actual ratio can vary greatly.