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ARTICLES:
Li-Hui Chen, Susan P. Baker, and Guohua Li
Graduated Driver Licensing Programs and Fatal Crashes of 16-Year-Old Drivers: A National Evaluation
Pediatrics 2006; 118: 56-62 [Abstract] [Full text] [PDF]
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[Read P3R] Caution urged in interpreting GDL findings
Robert D Foss   (4 July 2006)

Caution urged in interpreting GDL findings 4 July 2006
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Robert D Foss,
Research Scientist
University of North Carolina - Chapel Hill

Send letter to journal:
Re: Caution urged in interpreting GDL findings

rob_foss{at}unc.edu Robert D Foss

The Chen, Baker & Li (2006) article examining the effects of graduated driver licensing (GDL) on young driver fatalities is a valuable contribution to the adolescent injury prevention literature. However, it is important to avoid misinterpreting their findings when formulating teen driver policy.

This is a somewhat technical issue regarding researcher choices concerning details of how to conduct statistical analyses. In the present case, the matter is particularly important because the findings of single studies on young drivers are often quickly used by policy makers without attending to the larger body of literature on adolescent behavior in general, and young drivers in particular. The beneficial clarifying and filtering effect of research findings making their way to the general public through the professional research community is thus precluded.

The specific issue at hand is the authors' decision to use three months as the cut-point below which states were not considered to require a minimum duration learner permit (which allows only adult-supervised driving). The choice of three months is arbitrary. As a research decision, there is nothing wrong with that choice, but the findings should not be interpreted as suggesting that three months is sufficient to achieve the effect reported (although that is the clear, if unintended, implication). Nearly all states that fall into the category of "requiring at least three months supervised driving" in fact require six months or longer as the minimum learner period. Only one state that includes a minimum mandatory learner period requires as little as three months and only two others require less than six months; six months is by far the most common duration for a mandatory learner permit. Hence, the reported findings - with respect to learner permit periods - are more appropriately interpreted as resulting from learner periods of six months (or more) rather than three months.

The issue here is a case of the well-known and understood "grouping error," the resulting loss of information any time a continuum is treated as categorical or when many categories are collapsed into fewer categories. This is often necessary for analytic purposes, but information is inevitably lost in doing so. Researchers understand this and are cautious in their conclusions as a result, being careful not to attribute particular meaning to the precise cut-points and value ranges that have been chosen as a matter of necessity, rather than because of their inherent meaningfulness. In the present case, had the authors chosen 4 months, 5 months, or 6 months as their cut point, the results would have been essentially the same as were found using three months. Were any state policy makers to conclude that three months is sufficient to achieve the effects reported in this article, subsequently reducing a longer permit period to three months as a result, it would be a serious mistake, based on incorrect interpretation of the present findings.

Conflict of Interest:

None declared