In their paper “The contribution of alcohol to serious car crash injuries”, Connor et al. show a fairly strong association between alcohol consumption and the odds of getting into a serious car accident. Specifically, a person who consumes 2 or more drinks has 8 times the odds of getting into serious injury car crash in the following 6 hours when compared to a person who has not consumed alcohol. This is pretty intuitive, and the study seems fairly well done.

**But there’s something strange going on…**

One dimension of the findings that is curious (though not really discussed by the authors of the study) is that being a frequent drinker (e.g.,drinking 6-7 days a week) seems somewhat protective against the odds of getting into serious injury car crash. The following table from their paper is illustrative:

Compared to nondrinkers, persons who drink 6-7 days a week have a 0.3 times odds of being experiencing a serious car injury. This is roughly the equivalent of saying that frequent drinkers have ~ 70% reduction in the probability of getting into a serious car accident after controlling for demographic factors and the consumption of alcohol in the last 6 hours.

The real question is what is the *net* effect of drinking in the last 6 hours on frequent drinkers and non-drinkers. I can’t estimate this precisely based on the information presented in the study, but I can get a ballpark figure.

First, consider a simplified equation for the model they used:

log(p/(1-p)) = B0 + B1**x1** + B2**x2**

where B0 is the intercept, B1 is the coefficient for consuming alcohol in the last 6 hours and B2 is the coefficient for being a frequent drinker. The variables **x1** and **x2** are dichotomous (1 or 0), indicating whether or not a person consumed alcohol in the last 6 hours (**x1**) and whether or not a person is a frequent drinker (**x2**).

I will estimate B0 as 0.001, and the value of B1 and B2 can be found by transforming the odds ratios in the table. Using these inputs, I can derive model predicted log odds, convert these into probabilities, and then plot them on a graph:

This graph says two things. First, drinking increases the risk of car accidents, whether or not you are a frequent drinker. Second (and perhaps more controversially), frequent drinkers who drink are at relatively lower risk of getting injured in a serious car accident when compared to non-drinkers who drink. Of course, since non-drinkers drink infrequently (or perhaps never if their name means anything), the total public health impact of frequent drinkers is probably greater than non-drinkers. Nonetheless, these results could suggest that drinking and driving may be a *particular* concern among persons who are inexperienced as drinkers–perhaps due to lower alcohol tolerance, or less experience in judging their level of impairment.

There are a number of important caveats to consider here–including the fact that I do not have access to the original data. However, the results are not entirely unbelievable either; it seems plausible that all else being equal a frequently drinker may be less impaired by a small quantity of alcohol than an inexperienced drinker. This doesn’t mean drinking alcohol and operating a motor-vehicle is safe–it clearly isn’t–but just that the risks are complex.

**References**

Connor, Jennie, et al. “The contribution of alcohol to serious car crash injuries.” *Epidemiology* 15.3 (2004): 337-344.