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S P Morgan

Publications and source records attributed to S P Morgan.

At least 19 recordsLinked to original sources
PubMed2006

Comparison of methods for reducing the effects of scattering in spectrophotometry.

Light scattering provides a problem in optical spectroscopy as the relationship between attenuation and absorption in the presence of scattering is nonlinear. Three simple methods of reducing the effects of scattering and hence returning to an approximately linear relationship are considered in this paper, namely, extracting light that has maintained its original polarization state through subtraction of orthogonal polarization states, use of an added absorber, and spatial filtering. These can all be applied relatively easily to conventional spectrophotometers. However, there is an inevitable trade-off between the accuracy of the measurement and the signal-to-noise ratio as scattered light is rejected from the detector. It is demonstrated that polarization subtraction is the most efficient technique at selecting weakly scattered photons from a scattered light background as it enables the relationship between attenuation and absorption coefficient to become more linear while maintaining a higher number of detected photons. In practical use, the drawback of polarization subtraction over added absorber and spatial filtering methods is that a large dc background light level is maintained, which contributes to a higher shot noise. This means that when the scattering coefficient is high (micros > or = 7 mm(-1)) the added absorber method offers better performance for shot noise limited detection.

Journal Article↗
PubMed2002

Analysis of the spatial distribution of polarized light backscattered from layered scattering media.

The scattering of polarized light from a two layer scattering medium is investigated using Monte Carlo simulations. First order and normalized second order moments are used to analyze the spatial properties of the emerging light in different polarization states. Linearly and circularly polarized illumination is used to probe different depths. Absorption and layer thickness are varied and it is demonstrated that the determination of these values is aided by the inclusion of polarization information. The lateral and depth localization of light by polarization subtraction is also quantified. Potential applications of these techniques are burn depth and melanoma thickness measurements.

Biophysical Phenomena↗
PubMed2001

Success and future of demography: the role of data and methods.

Demography typifies paradigmatic success; that is, cumulative scientific work that has provided useful perspectives on a set of important questions. This success can be traced partly to the core subject matter of demography, which is relatively conducive to quantitative, observational science. The development of demography was further aided by extrinsic factors, such as the import of its data for government administration, for business purposes, and the import of demographic questions for social problems and public policy. These observations make suspect any simple projection of demography's success into the future or the transport of its experience to other disciplines.

Benchmarking↗
PubMed1999

Reexamining the link of early childbearing to marriage and to subsequent fertility.

Using data from the 1980, 1985, and 1990 Current Population Surveys, we show that the link between early fertility and nonmarital births has become stronger. Women who give birth earlier are increasingly likely to be unmarried. In contrast, we find a weaker association between first births at young (versus older) ages and (1) a rapid pace of subsequent childbearing and (2) higher completed fertility. We discuss possible causes and consequences of these changes.

Adolescent↗
PubMed1996

Racial differences in marriage and childbearing: oral history evidence from the South in the early twentieth century.

Using oral histories collected in 1938 and 1939 in the Southern United States, this article examines how African-Americans and whites viewed marriage and nonmarital childbearing. The authors document distinct racial differences in family norms and the sanctions that supported those norms. Giving birth outside a marital relationship was clearly not the stigmatizing event for African-Americans that it was for whites. The authors also found that African-Americans were more likely than whites to end marriages under similar conditions. These results suggest that debates about contemporary racial differences need to take into account the historical background, both cultural and demographic, of diverse groups.

Black or African American↗
PubMed1996

A decomposition of trends in the nonmarital fertility ratios of blacks and whites in the United States, 1960-1992.

We use a method of standardization and decomposition developed by Das Gupta to update Smith and Cutright's analysis of demographic factors responsible for increases in the nonmarital fertility ratio (illegitimacy ratio) among blacks and whites in the United States. We create standardized rates for each year between 1960 and 1992, and consistent, exhaustive decompositions of the nonmarital fertility ratio for any interval during this period in terms of four components: (1) the age distribution of women of reproductive age, (2) the proportion of women unmarried at each age, (3) the age-specific birth rates of married women, and (4) the age-specific birth rates of unmarried women. Nonmarital fertility ratios are much higher among blacks than among whites, but both increased monotonically from 1960 to 1992. During the last 10 years, each increased by nearly 10 percentage points. Increases in the proportion of women not married, at all ages, account for the preponderance of the increase in black nonmarital fertility ratios. Increasing rates of unmarried childbearing, however, have played a role during the most recent decade (1983-1992). For whites, from 1960 until 1975, declines in marital fertility were most important in producing increases in the proportion of children born out of wedlock. Since then, these proportions have increased primarily because of increases in unmarried women's birth rates, and secondarily because of declines in the proportion of women who are married. These trends are consistent with arguments that emphasize declining economic incentives to marry and reduced access to, and acceptability of, abortion.

Adolescent↗
PubMed1996

Education and the changing age pattern of American fertility: 1963-1989.

Using pooled data from the 1980, 1985, and 1990 Current Population Surveys, we describe fertility trends by age and education for the period 1963-1989. Interest focuses on whether the effects of education have changed across this period. We show that women with college degrees experienced dramatic shifts toward later ages of childbearing. This shift is consistent with arguments we develop about the increased opportunity for women to pursue careers and about changes in the availability of child care.

Adolescent↗
PubMed1992

Predicting childlessness for recent cohorts of American women.

Over 50% of the American women now in the midst of the childbearing years have never borne a child. These levels of childlessness for women in their 20s are the highest ones in a time series that spans most of the twentieth century. Will postponed parenthood be translated into very high levels of permanent childlessness? Or will these cohorts "catch-up" with a late fertility flurry in their 30s and 40s? The authors examine 3 projection strategies: one using women's stated fertility expectations, a second relying on the patterns of previous cohorts, and a third which posits that current rates will persist into the future. The predictive validity of these different projection strategies are tested with data for the 1980-87 period. They show that the projection based on current period rates performs well. Further, the authors argue that it better captures the first birth process than other models. They forecast levels of 20% childless for cohorts of white women born in the early 1960s. Recent trends for nonwhites are very different from those for whites: levels of 4% are forecast for nonwhite women.

Age Distribution↗
PubMed1992

African-American marriage in 1910: beneath the surface of census data.

This paper examines the quality of information about marital status, marital duration, and marriage order among African-American women in the U.S. Census of 1910. It compares the reported prevalence of widowhood to estimates of widowhood based on the mortality of black men and on the ages of women at first marriage. It also compares the reported distributions of duration of first marriage to estimates based on mortality and on age at first marriage. It concludes that census reports are subject to serious error. Widowhood is overreported, and marital turnover appears to have been faster than implied by census reports. The prevalance of "own children" is used to confirm these conclusions and to suggest motivations for misreporting.

Black or African American↗
PubMed1991

Recent trends in the timing of first births in the United States.

We use vital registration data published since 1979 to update trends in the timing of first births. Two important trends are documented. First, the likelihood that childless women over age 30 will have a first birth has increased since the 1970s. This change shows that women born in the 1950s are "catching up" on fertility postponed at younger ages. Second, racial differences in the timing of first births are very large. For those born in the 1950s, nonwhites have first births much earlier, and far fewer nonwhite than white women will remain permanently childless. In the second part of the paper, we use these data for recent years to assess earlier projections of childlessness based on cohort and period approaches. We also assess the accuracy of stated intentions to have no children.

Black or African American↗
PubMed1990

A comparison of three methods of managing fever in the neurologic patient.

Patients with neurological dysfunction can suffer fevers due to irritation of, or damage to the temperature-regulating center in the brain, or a combination of both. Although hypothermic therapy is a critical treatment nurses often perform, the most effective cooling method has not been studied in the neurological patient. The purpose of this quasi-experimental study was to compare the effectiveness of three methods of fever reduction, and the effect of each on patient shivering. Twenty-one adult febrile neurological patients were randomly placed in one of three temperature reduction groups. One of the temperature reduction protocols was initiated when a subject first became febrile. Rectal temperatures were taken every 15 minutes using an IVAC Temp-Plus thermometer until the subject's temperature returned to 100 degrees F. Subjects were observed for shivering throughout the procedures. The mean time necessary for the subject's temperature to return to 100 degrees F was calculated. A one-way analysis of variance of the time required to reach 100 degrees F was done to determine if a significant difference existed in rate of temperature reduction among the groups. The analysis revealed no statistically significant difference among the three methods, but a chi-square analysis indicated a significant relationship between shivering and the use of a hypothermia blanket.

Acetaminophen↗
PubMed1984

Measurement and replication: evaluating the consistency of eight U. S. fertility surveys.

This paper addresses the comparability of data from eight national fertility surveys conducted in the United States between 1955 and 1980. We examine the extent to which substantive conclusions about the levels and correlates of the timing of first birth vary across these data sources by comparing samples of women who were eligible to be included in more than one study. Results obtained from Ordinary Least Squares (OLS) and logistic regression analyses of several measures of timing indicate few study effects. Thus findings based on data from one survey can be replicated with comparable data from another. One notable exception is the relationship between contraceptive use prior to the first birth and the timing of that birth. The contraceptive variable appears to be poorly measured, leading to unstable estimates of the relationship. In general, our results suggest: (1) the feasibility of pooling data from the various surveys when modeling delayed childbearing; and (2) reasonable accuracy in assessing trends in the timing of first birth from multiple data sources.

Adolescent↗
PubMed1982

Parity-specific fertility intentions and uncertainty: the United States, 1970 to 1976.

A theoretical and analytic model of fertility intentions is proposed which treats "don't know" responses and other uncertain responses as distinct from more firm intentions. Methodologically, these analyses show that "don't know" responses need not be treated as missing data, but instead are both valid and meaningful responses. Furthermore, eliminating these uncertain respondents would have the negative effects of distorting across survey comparisons in intentions due to shifts in aggregate uncertainty, reducing the likelihood of accurately detecting shifts in fertility intentions, and lessening the representatives of the sample analyzed. Substantively, in conjunction with Morgan (1981), these results show that the sharp 1965-76 decline in the likelihood of intending more births at parities 2 through 5 occurred as women halted childbearing at minimal acceptable levels and postponed further childbearing. With time (or age), this delayed fertility became fertility about which the respondent was uncertain and, finally, fertility foregone. Since 1970, similar shifts are observed at parities 0 and 1, perhaps foreshadowing an increase in voluntary childlessness and one-child families.

Adolescent↗
PubMed1981

Intention and uncertainty at later stages of childbearing: the United States 1965 and 1970.

While births may be dichotomous, fertility intentions are not inherently so. Intentions are predictions about the future and, as such, are couched in considerable uncertainty. Ignoring this uncertainty hides much of what could be learned from data on fertility intentions. This paper presents a model which allows analysis of the full range of intentions. This paper presents a model which allows analysis of the full range of intentions. After selecting a sample of women in the later stage of childbearing (e.g., those who intend fewer than two additional children) from the 1965 and 1970 National Fertility Studies, it is shown that: (1) substantial portions of women at this stage of the reproductive life cycle were indeed uncertain of their parity-specific intention; (2) this certainty, like more firm intentions, varies by age and parity as the model predicts; and (3) there were significant shifts in the level of certainty between 1965 and 1970. Specifically, while intentions for third, fourth, and fifth births declined, more women "didn't know" if they intended to have another child or not. Among those not intending another child, more seemed uncertain of this intention in 1970 than did comparable women in 1965. In contrast, those intending another child seemed more certain. These changes in intention and uncertainty indicate that the observed decline in intended parity was tentative. Post-1970 evidence suggests that this tentative decline has become an equivocal one.

Adolescent↗