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As researchers developed developed predictive models which correlated BAC levels with varying degrees of deterioration impairment of driving-related skills, California DUI legislation making presumptions of DUI impairment relative to BAC was enacted. California DUI per se law was born.

Calculating BACs from drinks consumed, or back-calculating BACs to the time of driving became important in some California DUI cases. Impairment proved to be an inconvenient concept to prosecute California DUIs so they defined a California DUI offense by prohibiting a BAC in order to simplify prosecutions. California DUI per se law was established at .10% (later reduced to .08%).

This per se California Drunk Driving statute, now Vehicle Code Section 23152(b), was intended to remove entirely the idea of impairment by alcohol, substituting the BAC alone for any evidence regarding impairment of driving ability, thus focusing the attention on California BAC results as the "per se" proof of the offense.

California Vehicle Code Section 23152(b) makes it "unlawful for any person who has .08% or more, by weight, of alcohol in his or her blood to drive a vehicle."

In any California DUI prosecution under this statute, it is a rebuttable presumption that the person had .08% of alcohol in the blood at the time of driving the vehicle if the person had .08% in the blood at the time of the performance of a chemical test within 3 hours after the driving.

Since the California DUI per se statue criminalized a BAC at the time of driving, and most California drivers are tested at some time minutes or hours after the act of driving, it became important to "relate back" the BAC test result to the time of driving.

California permits evidence of a DUI chemical test result taken within hours of driving. "Extrapolation" is the "relate back" concept used to bring a test taken some time after driving into statutory or circumstantial relevance. Extrapolation is also used by California DUI defense attorneys to rebut the State of California's prima facie DUI case.

When a California DUI defense lawyer uses extrapolation to show a person's BAC is lower at the time of driving than it was at the time of test, that person should not be convicted of California's per se DUI statute.

Extrapolation of and from BACs or levels in the human body has been around since Erik M. P. Widmark developed his equations in the 1930's.

One key aspect of the Widmark equation has been calculating the correlation of a given BAC to a most likely number of drinks consumed. Another key aspect has been the calculation of a BAC to a given time from the BAC recorded at some after-occurring time.

There are at least (7) uncertain random variables which are involved in BAC estimates based on the Widmark Equation: (1) Weight, (2) rho factor (L/kg) [distribution of alcohol/body mass], (3) BAC at test time, (4) beta factor [=elimination rate], (5) time, (6) amount of alcohol per drink, and (7) amount of drinks consumed. While Widmark himself only considered the rho and beta factors to be uncertain, each of the factors is subject to variation.

The variability of each individual factor operates as a slight multiplier that affects its relationship to each other variable, and ultimately, the final result of the calculation.

Later research included co-variance terms, accounting e.g. for explaining that women with smaller rho values generally have higher beta values than men. The distribution variable has further been revised to reflect either total body water, or the traditional apparent volume of distribution in the body. Using breath testing in the U.S. assumes a partition ratio of 2100:1, which represents another variable term, should one need to compensate between the assumed value of 2100 and the more accurate 2300:1. (In the United Kingdom, the ratio is statutorily set at 1:2300.)

Other factors include the effects of food quantity and type, the limitations of the rho factor as a measure of true volume of distribution, first-pass metabolism or alcohol, gastric emptying, bioavailability or position on the alcohol time curve. Widmark assumes dosing at once on an empty stomach, complete absorption, and at least an hour and a half between the end of drinking and sampling BAC.

California DUI prosecutors want jurors to guess what the true BAC was in a San Diego DUI case by projecting backwards, using average alcohol absorption and elimination rates, but it's only an inaccurate guess based on assumptions. "Retrograde extrapolation"is a fancy name for trying to guess backwards. The San Diego DUI prosecutor offers BAC test evidence guessing one's BAC...back to the time of driving. The San Diego California DUI attorney has a recipe to respond. http://www.sandiegoduilawyer.com/Retrograde-Extrapolation.html

In San Diego California Drunk Driving trials, the Prosecution ultimately marches in their paid expert to "assume" what the San Diego DUI driver's BAC allegedly is. San Diego California DUI defense lawyer will show why it is not a good idea to "ASSUME" when trying to determine one's true BAC. Assuming through averages does not necessarily amount to proof of true BAC at time of driving beyond a reasonable doubt. http://www.sandiegoduilawyer.com/assume.html Widmark's formula has limitations but can be used professionally and with scientific validity if utilized properly.