General Estimates System (GES)
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U.S. DEPARTMENT OF TRANSPORTATION National Highway Traffic Safety Administration National Automotive Sampling System (NASS) General Estimates System (GES) Analytical User's Manual 1988-1999 NASS GES Analytical User’s Manual 1988 - 1999 U. S. Department of Transportation National Highway Traffic Safety Administration National Center for Statistics and Analysis Washington, D.C. 20590 TABLE OF CONTENTS Introduction ...................................................................1 GES Operations ................................................................2 GES Sample Design .............................................................3 GES SAS Files ................................................................5 Understanding the GES Imputation Process ...........................................7 National Estimates ..............................................................9 GES Variable List .............................................................10 GES Variables and Definitions: ....................................................15 Accident File ...........................................................16 Vehicle/Driver File .......................................................36 Person File .............................................................70 Appendices: A. Make/Model Designation ............................................83 B. V23 Accident Type Diagram ........................................116 C. Summary Statistics ...............................................117 D. Generalized Estimated Sampling Errors .................................119 E. Analytical Data Classification of Select GES Variables .....................129 Introduction One of the primary objectives of the National Highway Traffic Safety Administration (NHTSA) is to reduce the staggering human toll and property damage that motor vehicle traffic crashes impose on our society. Crashes each year result in thousands of lives lost, hundreds of thousands of injured victims, and billions of dollars in property damage. Good data are required to support the development, implementation, and assessment of highway safety programs aimed at reducing this toll. NHTSA uses data from many sources, including the National Automotive Sampling System General Estimates System (GES) which began operation in 1988. Providing data about all types of crashes involving all types of vehicles, the GES is used to identify highway safety problems areas, provide a basis for regulatory and consumer information initiatives, and form the basis for cost and benefit analyses of highway safety initiatives. The GES obtains its data from a nationally representative probability sample selected from the estimated 6.4 million police-reported crashes which occur annually. These crashes include those which result in a fatality or injury and those involving major property damage. Although various sources suggest that there are many more crashes that are not reported to the police, the majority of these unreported crashes involve only minor property damage and no significant personal injury. By restricting attention to police-reported crashes, the GES concentrates on those crashes of greatest concern to the highway safety community and the general public. This multi-year analytical user’s manual provides documentation on variables that are contained in the GES and other useful information that will enable the users to become familiar the data system. NASS GES Analytical User’s Manual, 1988-1999 1 GES Operations The GES is directed by the National Center for Statistics and Analysis, which is a component of Research and Development in NHTSA. The data are obtained by GES data collectors in 60 geographic sites across the United States. These data collectors make weekly, biweekly, or monthly visits to approximately 400 police agencies within the 60 sites. During the visit, the data collectors list all police traffic crash reports (PARs) not previously listed and then select a sample of the listed PARs. The collector obtains copies of these selected PARs and sends them to a contractor for coding. Trained personnel interpret and code data directly from the PARs onto an electronic file. To protect individual privacy, no personal information such as names, addresses, specific crash location, etc., is coded. During data coding, the data are checked for validity and consistency. After the data file is created, quality checks are performed on the data. When these are completed, the electronic data are made available to governments, researchers, motor vehicle manufacturers, insurance companies, and others. The GES data are also used to respond to requests from the international and national highway safety communities, state and local government, the Congress, federal agencies, research organizations, industry, the media, and private citizens. Currently, the 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, and 1999 data files are available. For information on obtaining a copy of any of these data files, contact: Marjorie Saccoccio, DTS-44 DOT/Transportation Systems Center Kendall Square Cambridge, MA 02142 or GES data can be obtained by downloading any of the published files from the Internet, at ftp://www.nhtsa.dot.gov/ges. The files are available in SAS and sequential ASCII file formats. 2 NASS GES Analytical User’s Manual, 1988-1999 GES Sample Design The police accident reports (PARs) from which GES data are coded are a probability sample of police-reported crashes that occurred in the United States. Since each crash that occurred in the survey year had a chance of being selected, the design makes it possible to compute not only national estimates but also probable errors associated with the estimates. The selection of the sample of PARs for the GES was accomplished in three stages. The first stage is a sample of geographic areas, called Primary Sampling Units (PSUs), from across the United States. A PSU is either a central city, a county surrounding a central city, an entire county, or a group of contiguous counties. The U.S. was divided into 1,195 of these PSUs. The PSUs were then grouped into 14 categories according to the following geographic regions and types of PSUs: ' Geographic Region: Northeast, Midwest, South, and West ' Type: Large Central City, Large Suburban Area, and All others. The second stage of the design is a sample of police jurisdictions within the geographic areas. In most areas, the number of police jurisdictions is more than can reasonably be visited by a data collector. All jurisdictions within a PSU were enumerated and the number of crashes investigated by each was determined. A probability sample of jurisdictions within each PSU was selected with probability proportional to the number of crashes investigated, i.e., as the number of crashes investigated increased, the probability of selecting that jurisdiction increased. An average of six or seven police jurisdictions were selected within each area. The third and final stage is the selection of PARs within the sampled police jurisdictions. The PARs are grouped, or stratified, into one of four groups by the data collector: ' Group 1: NASS crashes involving at least one passenger vehicle, i.e., a passenger car, sport utility vehicle, pickup truck or van) towed due to damage from the crash scene and no medium or heavy trucks are involved; ' Group 2: NASS crashes not qualifying for Group 1 involving at least one medium or heavy truck in which a vehicle was towed due to damage or at least one involved person had a police-reported injury of “K”, ”A”, ”B”, or “C”; ' Group 3: NASS crashes not qualifying for Group 1 or 2 in which none of the vehicles involved in the crash was a medium or heavy truck and at least one person involved in the crash had a police-reported injury of “K”, ”A”, NASS GES Analytical User’s Manual, 1988-1999 3 or ”B”; and, ' Group 4: NASS crashes not qualifying for Group 1, 2 or 3 . No one in the crash can receive a “K”, ”A”, or ”B” injury. Within each of these groups a systematic sample of crashes is selected, based on different sampling ratios. In some very large police jurisdictions the number of police investigated crashes is too many for reasonable listing. In these jurisdictions the data collector will list a subsample of PARs, with those listed depending on the PAR number. The data collector obtains copies of the selected PARs and sends them to the NASS zone centers for quality review and processing. The zone centers then code the selected PARs into a common format and create an electronic file. In 1999 approximately 54,000 PARs were sampled and coded. A thorough discussion of the sample design can be found in the National Accident Sampling System General Estimates Technical Note, DOT HS 807 796. For a copy, write: Department of Transportation National Highway Traffic Safety Administration National Center for Statistics and Analysis, NRD-31 400 Seventh Street SW Washington, D.C. 20590 4 NASS GES Analytical User’s Manual, 1988-1999 GES SAS Files Overview Once the GES data are coded onto an electronic file, a Statistical Analysis System (SAS) data file is created. SAS is a software system for data analysis. The SAS data file for GES consists of three individual data sets: the Accident File, Vehicle/Driver File, and Person File. The Accident File contains information describing environmental conditions and roadway characteristics at the time of the crash. The Vehicle/Driver File contains information describing the vehicles involved in the crash and their drivers. It