Environmental stability graphite/PMR-15 composites

During the past few years the Boeing Company was screening graphite composites for use in hot areas of engine nacelle structure. Structural and thermal analyses have shown that there is the potential for a 25 to 30 percent weight savings by using a graphite polyimide (Gr/PI) composite material in this type of structure. Work conducted on the NASA CASTS program (Composites for Advanced Space Transportation Systems) amply demonstrated the capability of Graphite/PMR-15 for short term service (125 hours) at temperatures up to 589 K (600 F). In addition, the CASTS program demonstrated that large structures could be fabricated using Gr/PMR-15. In commercial applications, however, the requirement exists for long term service capability (tens of thousands of hours) at temperatures ranging upwards from 449 K (350 F). The results of Graphite/PMR-15 materials characterization efforts conducted at Boeing are presented with emphasis on materials properties after isothermal aging at temperatures of 449 K (350 F) and above

Using Discontinuous Eligibility Rules to Identify the Effects of the Federal Medicaid Expansions on Low Income Children

Despite intensive scrutiny, the effects of Medicaid expansions on the health insurance status of low-income children remain controversial. We re-examine the effects of the two largest federally-mandated expansions which offered Medicaid coverage to low-income children in specific age ranges and birth cohorts. We use a regression discontinuity approach, comparing Medicaid enrollment, private insurance coverage, and overall insurance coverage on either side of the age limits of the laws. We conclude that the modest impacts of the expansions on health insurance coverage arose because of very low takeup rates of the newly available coverage, rather than from crowd-out of private insurance coverage.

Potential bronchoconstrictor stimuli in acid fog.

Acid fog is complex and contains multiple stimuli that may be capable of inducing bronchoconstriction. These stimuli include sulfuric and niric acids, the principal inorganic acids present; sulfites, formed in the atmosphere as a reaction product of sulfur dioxide and water droplets; fog water itself, a hypoosmolar aerosol; the organic acid hydroxymethanesulfonate, the bisulfite adduct of formaldehyde; and gaseous pollutants, e.g., sulfur dioxide, oxides of nitrogen, ozone. Given this complexity, evaluation of the respiratory health effects of naturally occurring acid fog requires assessment of the bronchoconstrictor potency of each component stimulus and possible interactions among these stimuli. We summarize the results of three studies that involve characterization of the bronchoconstrictor potency of acid fog stimuli and/or their interaction in subjects with asthma. The results of the first study indicate that titratable acidity appears to be a more important stimulus to bronchoconstriction than is pH. The results of the second study demonstrate that sulfite species are capable of inducing bronchoconstriction, especially when inhaled at acid pH. The results of the third study suggest that acidity can potentiate hypoosmolar fog-induced bronchoconstriction

Using Discontinuous Eligibility Rules to Identify the Effects of the Federal Medicaid Expansions on Low Income Children

This paper exploits the discrete nature of the eligibility criteria for two major federal expansions of Medicaid to measure the effects on Medicaid coverage, overall health insurance coverage, and the probability of visiting a doctor. The '100 percent' expansion, effective in 1991, extended Medicaid eligibility to children born after September 30, 1983 in families below the poverty line. We estimate that this law led to about a 10 percentage point rise in Medicaid coverage for children born just after the cutoff date, and a similar or slightly smaller rise in overall health insurance. It also increased the fraction of children in the newly eligible group with a doctor visit in the previous year. The '133 percent' expansion, effective in 1990, extended Medicaid to children under 6 in families with incomes below 133 percent of the poverty line. This law had relatively small effects on Medicaid coverage for children near the eligibility limits, and little or no effect on health insurance coverage.

Evolution of size-dependent flowering in Onopordum illyricum: A quantitative assessment of the role of stochastic selection pressures

We explore the evolution of delayed, size-dependent reproduction in the monocarpic perennial Onopordum illyricum, using a range of mathematical models, parameterized with long-term field data. Analysis of the long-term data indicated that mortality, flowering, and growth were age and size dependent. Using mixed models, we estimated the variance about each of these relationships and also individual-specific effects. For the held populations, recruitment was the main density-dependent process, although there were weak effects of local density on growth and mortality Using parameterized growth models, which assume plants grow along a deterministic trajectory, we predict plants should flower at sizes approximately 50% smaller than observed in the field. We then develop a simple criterion, termed the "1-yr look-ahead criterion," based on equating seed production now with that of next year, allowing for mortality and growth, to determine at what size a plant should flower. This model allows the incorporation of variance about the growth function and individual-specific effects. The model predicts flowering at sizes approximately double that observed, indicating that variance about the growth curve selects for larger sizes at flowering. The 1-yr look-ahead approach is approximate because it ignores growth opportunities more than 1 yr ahead. To assess the accuracy of this approach, we develop a more complicated dynamic state variable model. Both models give similar results indicating the utility of the 1-yr look-ahead criterion. To allow for temporal variation in the model parameters, we used an individual-based model with a generic algorithm. This gave very accurate prediction of the observed flowering strategies. Sensitivity analysis of the model suggested that temporal variation in the parameters of the growth equation made waiting to flower more risky, so selected for smaller sizes at flowering. The models clearly indicate the need to incorporate stochastic variation in life-history analyses

The Effect of Medicaid Expansions for Low-Income Children on Medicaid Participation and Private Insurance Coverage : Evidence from the SIPP

We examine Medicaid enrollment and private coverage loss following expansions of Medicaid eligibility. We attempt to replicate Cutler and GruberÂ’s (1996) results using the Survey of Income and Program Participation, and find smaller rates of take-up and little evidence of crowding out. We find that some of the difference in results can be attributed to different samples and recall periods in the data sets used. Extending the previous literature, we find that take-up is slightly increased if a childÂ’s siblings are eligible and with time spent eligible. Focusing on children whose eligibility status changes during the sample, we estimate smaller take-up effects. We find little evidence of crowding out in any of our extensions.

Did Expanding Medicaid Affect Welfare Participation?

Using data from the 1988-1996 Current Population Surveys (CPS), we re-examine the evidence presented in Yelowitz (1995) showing that expansions in Medicaid eligibility for children were associated with increased labor force participation and reduced participation in Aid to Families with Dependent Children (AFDC) among single mothers. We find that Yelowitz's results were the result of two factors. First, he imposed a strong restriction on the parameter estimates that is not predicted by theory and is rejected in the CPS data. Second, he used only one of the two income tests that families must pass to be eligible for AFDC, resulting in higher imputed AFDC breakeven income levels for larger families. Once these problems are addressed, the Medicaid income limits have no significant effect on AFDC participation. The AFDC income limits, however, are significantly related to welfare and labor force participation in both his original sample and the entire 1988-1996 sample.

The Effect of Medicaid Expansions for Low-Income Children on Medicaid Participation and Insurance Coverage: Evidence from the SIPP

Increased availability of public health insurance for children has led to two potentially contradictory concerns for public policy: that expanded availability of public insurance may lead families to decline private insurance and that additional public coverage may not reach many uninsured children. We examine these two concerns using data from the 1987-1993 Surveys of Income and Program Participation. Using static models we find that the expansions resulted in increased Medicaid coverage, although the estimates of take-up are smaller than estimates from previous research. We find little evidence of a negative relationship of any significant magnitude between eligibility for Medicaid and private coverage. We also find that children who have been eligible for Medicaid longer are more likely to be enrolled in Medicaid but no more likely to have lost private coverage. Including individual fixed effects reduces the magnitude of the estimated take-up effect, while the fixed effects estimates for the private insurance regression become negative and marginally statistically significant in some specifications. Simple dynamic models of insurance choice show that insurance choice is quite persistent. The estimated long run impact of eligibility in the dynamic models is larger than the estimate from the static models, while the immediate impact of expanded Medicaid eligibility from the dynamic models is smaller than the estimated effect from the static models.

The Impact of Public Health Insurance on Labor Market Transitions

An often-cited difficulty with moving low-income families out of welfare and into the labor force is the lack of health insurance in many low-wage jobs. Consequently, many low-income household heads may be reluctant to leave welfare and thereby lose health insurance coverage for their children. The expansions in the Medicaid program to cover low-income children and pregnant women who are not eligible for cash benefits may help alleviate the problem by allowing disadvantaged household heads to accept jobs which do not provide health insurance. We use a discrete time (monthly) hazard rate model and data from several panels of the Survey of Income and Program Participation to assess whether expansion of public health insurance to cover children of working parents contributes to increase transitions out of welfare and into employment and reduce transitions into welfare and out of employment. We model spells in progress and spells that start during the sample separately, which allows us to assess the effect on long-term welfare recipients. We find some evidence that expanded Medicaid eligibility for children leads single mothers to exit welfare more quickly; however the effects are not robust to the inclusion of year effects. In addition, the effect appears to be strongest and most consistent among long-term recipients (as proxied by recipients who begin the sample on welfare). We find less evidence of an effect of expanded Medicaid eligibility on transitions into welfare. A somewhat surprising finding is that higher AFDC income limits also appear to have little effect on the probability of such transitions.