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This paper decomposes the conventional measure of selection bias in observational studies into three components. The first two components are due to differences in the distributions of characteristics between participant and nonparticipant (comparison) group members: the first arises from differences in the supports, and the second from differences in densities over the region of common support. The third component arises from selection bias precisely defined. Using data from a recent social experiment, we find that the component due to selection bias, precisely defined, is smaller than the first two components. However, selection bias still represents a substantial fraction of the experimental impact estimate. The empirical performance of matching methods of program evaluation is also examined. We find that matching based on the propensity score eliminates some but not all of the measured selection bias, with the remaining bias still a substantial fraction of the estimated impact. We find that the support of the distribution of propensity scores for the comparison group is typically only a small portion of the support for the participant group. For values outside the common support, it is impossible to reliably estimate the effect of program participation using matching methods. If the impact of participation depends on the propensity score, as we find in our data, the failure of the common support condition severely limits matching compared with random assignment as an evaluation estimator. This paper uses data from a large-scale social experiment conducted on a prototypical job training program to decompose conventional measures of selection bias into a component corresponding to selection bias, precisely defined, and into components arising from failure of a common support condition and failure to weight the data appropriately. We demonstrate that a substantial fraction of the conventional measure of selection bias is not due to selection, precisely defined, and we conjecture that this is a general finding. We find that the conventional measure of selection bias is misleading. We also provide mixed evidence on the effectiveness of the matching methods widely used for evaluating programs. The selection bias remaining after matching is a substantial percentageoften over 100%-of the experimentally estimated impact of program participation.Our analysis is based on the Roy (1) model of potential outcomes, which is identical to the Fisher (2) model for experiments and to the switching regression model of Quandt The parameter of interest considered in this paper is the mean effect of treatment on the treated. It is not always the parameter of interest in evaluating social programs [see Heckman and Robb (6), Heckman (7), Heckman and Smith (8) and Heckman et al. (9)], but it is commonly used. It gives the expected gain from treatment for those who receive it. For covariate vector X, it is defined asSometimes interest focuses on the average impact for X in some region K, e.g.,in the definition of ⌬(X) can be i...

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The characterization of the Brillouin-loss spectrum of single-mode fibers with very short (<10-ns) pulses has been studied. It was found that the Brillouin-loss signal intensity is linearly related to the duration of the pump pulse used to obtain the spectrum. In contrast with the uniform trend of the signal, three distinct behaviors were observed in the spectral linewidth. At long pulse durations the linewidth was constant at approximately 40 MHz. Pulse durations of the order of the phonon lifetime resulted in a broader spectrum, reaching a maximum width of ~100 MHz at 5 ns. Reducing the pulse duration further resulted in a sudden narrowing of the Brillouin line.

The reduced Raman spectrum of amorphous Te consists of broad maxima at 90 and 157 cm-'. The spectrum reflects the vibrational density of states and is interpretable in terms of disordered chains. The highest vibrational energies of the disordered chains are higher than the corresponding energies of the ordered chains in crystalline Te. The higher vibrational energies in the amorphous form are due to the reduced effectiveness of long range forces which tend to depress vibrational energies in the crystalline form.Das reduzierte Ramanspektrum von amorphem Tellur besteht aus einem breiten Maximum bei 90 und bei 157 cm-l. Das Spektrum gibt die Vibrationszustandsdichten wiedeund laDt sich mit der Annahme fehlgeordneter Ketten verstehen. Die hochsten Vibrationsr energien der fehlgeordneten Ketten liegen hoher als die entsprechenden Energien der geordneten Ketten in kristallinem Tellur. Die hoheren Vibrationsenergien der amorphen Form lassen sich auf die reduzierte Wirksamkeit weitreichender Kriifte zuriickfiihren, die zu einer Reduzierung der Vibrationsenergien in der kristallinen Form fiihren.

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The Raman spectrum of glassy carbon has been measured. Two broad lines are observed at 1340 and 1590 cm−1. These results are consistent with the turbostratic structure, which has been suggested for glassy carbon, with a particle size La of approximately 30 Å.

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