An Analysis of the Iowa Child Passenger Safety SurveyBased on Generalized Linear Mixed Models Joseph Cavanaugh and Eric Chen Department of Biostatistics The University of Iowa New York State Psychiatric Institute Columbia University February 26, 2009
Children and Motor Vehicle Accidents According to the National Safe Kids Coalition, motor vehicle accidents are the leading cause of death in the United States among children from 3 to 14 years of age. In Iowa, approximately 40 children each year are killed in motor vehicle accidents, one every 9 days. (Iowa Department of Public Safety, 2004)
Children and Motor Vehicle Accidents Child safety seats reduce the risk of death by 71% for infants, and by 54% for children aged 1 to 4 years. (National Highway Traffic Safety Administration, 2005) For children aged 4 to 7 years, booster seats reduce injury risk by 59% compared to seat belts alone. (Durbin et al., 2003) The proper use of child safety seats, booster seats, and seat belts is the best protection available to keep children safe in motor vehicles.
Iowa History In 2001, the National Safe Kids Coalition graded each state’s child restraint law. Iowa received an "F", ranking 46 out of 51 (50 states and the District of Columbia). In July of 2004, Iowa’s Child Restraint Law was strengthened based on recommendations from the National Highway Traffic Safety Administration and the American Academy of Pediatrics. The revised law included an 18-month education phase prior to full enforcement of the new requirements.
Iowa History To measure compliance with the law and to direct educational efforts, observational restraint usage surveys have been conducted annually since 1988. These child passenger safety surveys are funded by the Governor’s Traffic Safety Bureau (GTSB). The GTSB has contracted with the University of Iowa Injury Prevention Research Center (IPRC) to conduct the surveys since 1996.
IRPC Child Passenger Safety Survey In 2004, the IPRC redesigned the survey in conjunction with the implementation of the new law. The sampled communities, and targeted sample sizes within these communities, were selected so that the sample would resemble the state population in terms of its rural and urban composition. The annual targeted sample size was set at 3,000. The data is collected by three trained surveyors.
IPRC Child Passenger Safety Survey The new data collection protocol requires the surveyor to approach the driver in the parking lot of a convenience store and to ask for his/her participation.
A card is given to the driver explaining the study.
The restraint status of each child is directly observed.
The restraint status of the driver (belted / not belted) and the vehicle type (truck, car, van, SUV) are also recorded.
No identifying information (e.g., names, license plate numbers) is collected.
An annual report summarizing the survey results is presented to the Iowa state legislature.
IPRC Child Passenger Safety Survey This presentation summarizes our first attempt to formally analyze the data using statistical modeling. Our goal is to characterize the factors that are associated with proper restraint use. We will employ the framework of generalized linear mixed models (GLMM).
Iowa Law Requirements of the current Iowa law:
Children must ride in an appropriate rear-facing child safety seat until one year of age and at least 20 pounds.
Children must ride in a child safety seat or a booster seat through the age of 5 years.
Children ages 6 through 10 must ride in a booster seat or use a seat belt.
Child restraint driver education chart: Iowa Law
Rear-Facing Safety Seat From birth up to 1 year old, the child should be put in a rear-facing safety seat.
Front-Facing Safety Seat From 1 through 5 years old, the child should be put in a safety seat or a booster seat.
Booster Seat / Seat Belt From 6 through 10 years old, the child should be put in a booster seat or restrained with a seat belt.
Problems with Restraint Use Two major problems with restraint use:
Many children are unrestrained, especially children from 6 through 10 years old.
Many toddlers (1 through 5 years old) are restrained with a seat belt as opposed to a booster or safety seat.
Use of Restraint Devices (2005-2007) Of the 2042 improperly restrained children,
37.5% (766/2042) were children from 1 through 5 years old who were wearing a safety belt,
Sampling for IPRC Study The survey data is compiled by collecting samples from 36 Iowan communities or sites. The sampled sites, and targeted sample sizes within these sites, were selected so that the sample would resemble the distribution of the state population over four urban / rural strata.
Data Structure and Model We model the response variable as a function of the explanatory variables using the framework of generalized linear mixed models (GLMM). Our model is formulated to account for two sources of correlation.
Correlation among responses collected within the same site.
Spatial correlation between sites based on the proximity between the sites.
An important source of correlation that could not be modeled (since the data was not collected) is the correlation among responses collected within the same vehicle.
Spatial Correlation Residual mean based on fitted generalized linear model (without inclusion of urban/rural covariate)
Random Effect Covariance The random effect included in the GLMM accounts for within and between site correlations. An isotropic exponential spatial covariance structure is assumed for the random effect.
The covariance between two sites is given by
where is the Euclidean distance between the sites.
Note that the covariance decreases as the distance between sites increases.
The effective range, corresponds to the distance beyond which the correlations fall below 0.05.
Spatial Variance-Covariance Structure Site 1 Site 2 Site 3 Site 1 Site 2 Site 3
GLMM Structure and GLIMMIX Code proc glimmix; class variables; model <resp> = <fixed effects> / dist= link= ; random <random effects> / <options>; run; Type=sp(exp) (lat long);
Spatial Random Effect Euclidean distance is calculated using latitude and longitude. Covariance parameter estimates: The effective range is estimated by
Spatial Random Effect The output suggests that a minor degree of spatial correlation exists between nearby sites.
Proper Restraint Use by Year The data shows an increase in the use of proper restraints for child passengers.
Proper Restraint Use vs. Driver Belted Status 2005-2007
Proper Restraint Use by Urban/Rural Status 2005-2007
Proper Restraint Use by Vehicle Size Year 2005-2007
Fixed Effects Estimates from GLMM Fit Significant odds ratios:
Conclusions The data exhibits some degree of spatial correlation. In the multivariable model, rural/urban status is not statistically significant. Compliance with the restraint laws has been increasing; the increases are both statistically significant and of practical importance.
Conclusions Drivers are most cautious with infants (age 0 to 1).
The odds of an infant being properly restrained are about 8 times as great as the odds of a young child (aged 6 through 10) being properly restrained.
For toddlers (age 1 through 5), restraint laws are not fully understood.
The odds of a toddler being properly restrained are half as great as the odds of a young child (aged 6 through 10) being properly restrained.
Drivers who are belted are more likely to use proper restraints for their children.
If the driver is belted, the odds of a child passenger being properly restrained are about 8 times as high as the odds if the driver is not belted.
The larger the cab size of the vehicle, the more likely that child passengers are to be properly restrained.
For vehicles with large cabs, the odds of a child passenger being properly restrained are about 3 times as high as the odds for vehicles with small cabs.
For vehicles with medium cabs, the odds of a child passenger being properly restrained are about 1.5 times as high as the odds for vehicles with small cabs.
Vehicle type is a potential risk factor. There is a statistically significant improvement in proper restraint use from 2005 to 2007.