Difference-indifferences (DID) research designs usually rely on variation of treatment timing such that, after making an appropriate parallel trends assumption, one can identify, estimate, and make inference about causal effects. In practice, however, different DID procedures rely on different parallel trends assumptions (PTAs), and recover different causal parameters. In this paper, we focus on staggered DID (also referred as event studies) and discuss the role played by the PTA in terms of identification and estimation of causal parameters. We document a "robustness" versus "efficiency" trade-off in terms of the strength of the underlying PTA and argue that practitioners should be explicit about these trade-offs whenever using DID procedures. We propose new DID estimators that reflect these trade-offs and derive their large sample properties. We illustrate the practical relevance of these results by assessing whether the transition from federal to state management of the Clean Water Act affects compliance rates.

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Gasoline content regulations are designed to curb pollution and improve health, but their impact on health has not been quantified. By exploiting both the timing of the regulation and spatial variation in children’s exposure to highways, I estimate the effect of gasoline content regulation on pollution and child health. The introduction of cleaner-burning gasoline in California in 1996 reduced asthma admissions by 8 percent in high exposure areas. Reductions are greatest for areas downwind from highways and heavy traffic areas. Stringent gasoline content regulations can improve child health, and may diminish existing health disparities.

As health care provision in the United States shifts to primary care settings, it is vital that new models of occupational health services be developed that link clinical care to prevention. The model program described in this paper was developed at the Union Health Center (UHC), a comprehensive health care center supported by the International Ladies Garment Workers Union (now the Union of Needletrades, Industrial and Textile Employees) serving a population of approximately 50,000 primarily minority, female garment workers in New York City. The objective of this paper is to describe a model occupational medicine program in a union‐based comprehensive health center linking accessible clinical care with primary and secondary disease prevention efforts. To assess the presence of symptoms suggestive of occupational disease, a health status questionnaire was administered to female workers attending the UHC for routine health maintenance. Based on the results of this survey, an occupational medicine clinic was developed that integrated direct clinical care with worker and employer education and workplace hazard abatement. To assess the success of this new approach, selected cases of sentinel health events were tracked and a chart review was conducted after 3 years of clinic operation. Prior to initiation of the occupational medicine clinic, 64% (648) of the workers surveyed reported symptoms indicative of occupational illnesses. However, only 42 (4%) reported having been told by a physician that they had an occupational illness and only 4 (.4%) reported having filed a workers' compensation claim for an occupational disease. In the occupational medicine clinic established at the UHC, a health and safety specialist acts as a case manager, coordinating worker and employer education as well as workplace hazard abatement focused on disease prevention, ensuring that every case of occupational disease is treated as a potential sentinel health event. As examples of the success of this approach, index cases of rotator cuff tendonitis, lead poisoning, and formaldehyde overexposure in three patients and their preventative workplace follow‐up, affecting approximately 150 workers at three worksites, are described. Work‐related conditions diagnosed during the first 3 years of clinic operation included cumulative trauma disorders (141 cases), carpal tunnel syndrome (47 cases), low back disorders (33 cases), lead poisoning (20 cases), and respiratory disease (9 cases). This pilot project represents a new model for effective integration of clinical care and occupational disease prevention efforts within a primary care center. It could serve as a prototype for development of such services in other managed and primary care settings. Am. J. Ind. Med. 31:263–273, 1997. © 1997 Wiley‐Liss, Inc.