Smokers lose at least one decade of life expectancy, as compared with those who have never smoked. Cessation before the age of 40 years reduces the risk of death associated with continued smoking by about 90%.

in the invited group (49 in the invited group vs 95 in the control group). After a normal first scan, 59 ruptured AAAs occurred, and 80% of these were fatal. Ruptured AAA after a normal first scan had a marked increase after 8 years of follow-up. Among the measured baseline aortic diameters, about half of those initially screened as normal and subsequently having ruptured AAA were in the initial screening range of aortic diameters of 2.5 to 2.9 cm.Comment: There are at least two findings that should be emphasized here. The first is that screening for AAA reduces all-cause mortality. The second is that screening is not perfect, and some patients will still develop aneurysms and die from them after an initial so-called negative screening study. The MASS trial measured AAAs with inner-to-inner wall measurements of diameter by ultrasound imaging. Theoretically, because this results in a smaller diameter measurement, use of the Society for Vascular Surgery guidelines for measuring abdominal aortas from outer-to-outer wall could potentially save some ruptures without rescreening of "normals" or an increase in the number of initial screening studies.

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In observational studies the assignment of units to treatments is not under control. Consequently, the estimation and comparison of treatment effects based on the empirical distribution of the responses can be biased since the units exposed to the various treatments could differ in important unknown pretreatment characteristics, which are related to the response. An important example studied in this article is the question of whether private schools offer better quality of education than public schools. In order to address this question we use data collected in the year 2000 by OECD for the Programme for International Student Assessment (PISA). Focusing for illustration on scores in mathematics of 15-years old pupils in Ireland, we find that the raw average score of pupils in private schools is higher than of pupils in public schools. However, application of a newly proposed method for observational studies suggests that the less able pupils tend to enroll in public schools, such that their lower scores is not necessarily an indication of bad quality of the public schools. Indeed, when comparing the average score in the two types of schools after adjusting for the enrollment effects, we find quite surprisingly that public schools perform better on average. This outcome is supported by the methods of instrumental variables and latent variables, commonly used by econometricians for analyzing and evaluating social programs.