What is Retrograde Extrapolation?

In some cases a person's blood alcohol level may not be high enough to attempt to punish him or her for driving with a proscribed level. In these cases the prosecutor will try an "relate back" the BAC to the time of driving in an effort to show the BAC was higher at the time of drving versus the time the test was administered. Some believe that this is nothing more than an educated guestimate. To do this the DA will use an expert to use various "known" facts and assume the other unknowns. This practice is frowned upon by many DUI scientists yet many Judges will allow the evidence despite crticism by those in the scientific community.