A statistical model of tracking

A method is proposed to model individual patterns of growth over time by linear combinations of optimally chosen weighted orthogonal vectors. The goal is to distinguish individuals who track from nontrackers. Nontrackers are defined as those who follow different, usually more complex, growth patterns than trackers. Thus, nontrackers require more vectors than do trackers in modeling their longitudinal observations. A method of specifying the class-specific vectors and individual weights is demonstrated. When the proportion of nontrackers in the population is small, a modified form of the Akaike maximum entropy criterion is used to select the number of vectors appopriate for each person and also to classify each person into a tracking category. When the proportion of nontrackers is large, the modified Akaike criterion together with scatterplots of the growth curve weights are needed to distinguish trackers from nontrackers. The apprach is illustrated with longitudinal observations of height measured in an epidemiologic survey of children.     

A Self-Image Inventory: Its Theoretical Background,Reliability and Validity*

Wylie (1974) points out that there is a lack of adequate theoretical development in the investigation of the self-concept. The result is a proliferation of operational definitions with no clear understanding of the relationship between operations, between constructs, and between the constructs and their operationalizations. In this article we make explicit the theoretical component of self to which this Self-image Inventory relates. The article presents the inventory and twenty years of research experience with it. The inventory is found to be both reliable and valid for measuring the self-image as a role specific component of self.     

COHORT DIFFERENCES IN THE TRANSITION TO MOTHERHOOD:

Although research has found that educational attainment is a major predictor of age at first childbirth, this association may not be consistent across cohorts. In this article, we examine the adult transitions of four cohorts of married white women (N = 306), born between 1907 and 1933, including in our life course model not only education but also employment before marriage. As hypothesized, employment before marriage is the best predictor of the age at first childbirth among women entering adulthood prior to the end of World War II. In contrast, women in the early postwar years were moderately more likely to postpone childbearing due to educational pursuits.     

Confidence limits for estimates of totals from stratified samples, with application to medicare Part B overpayment audits

Superpopulation models are proposed that should be appropriate for modelling sample-based audits of Medicare payments and other overpayment situations. Simulations are used to estimate the coverage probabilities of confidence intervals formed using the standard Stratified Expansion and Combined Ratio estimators of the total. Despite severe departures from the usual model of normal deviations, these methods have actual coverage probabilities reasonably close to the nominal level specified by the US government's sampling guidelines. An exception occurs when all claims from a single sampling unit are either completely allowed, or completely denied, and for this situation an alternative is explored. A balanced sampling design is also examined, but shown to make no improvement over ordinary stratified samples used in conjunction with ratio estimates.     

One-Hit Models of Carcinogenesis: Conservative or Not?

One-hit formulas are widely believed to be "conservative" when used to analyze carcinogenesis bioassays, in the sense that they will rarely underestimate risks of cancer at low exposures. Such formulas are generally applied to the lifetime incidence of cancer at a specific site, with risks estimated from animal data at zero dose (control), and two or more additional doses that are appreciable fractions of a maximum tolerated dose. No empirical study has demonstrated that the one-hit formula is conservative in the sense described. The Carcinogenesis Bioassay Database System contains data on 1212 separate bioassays of 308 chemical substances tested at exactly three evaluable doses. These provided sufficient data to examine 8432 specific combinations of cancer site with sex, species, and chemical. For each of these we fitted a one-hit formula to the zero and maximum dose data points, then examined the relation of the fitted curve to the incidence rate observed at the mid-dose, with and without adjustment for intercurrent mortality. Both underestimates and overestimates of risk at mid-dose occurred substantially more often than expected by chance. We cannot tell whether such underestimates would occur at lower doses, but offer six biological reasons why underestimates might be expected. In a high percentage of animal bioassays, the one-hit formula is not conservative when applied in the usual way to animal data. It remains possible that the one-hit formula may indeed be conservative at sufficiently low doses (below the observational range), but the usual procedure, applied to the usual dose range, can be nonconservative in estimating the slope of the formula at such low doses. Risk assessments for regulation of carcinogens should incorporate some measure of additional uncertainty.     

“Oh,No, I'm Not Infertile”: Culture,Support Groups,and the Infertile Identity

Abstract

Using in-depth interviews with forty subfecund women, I explore how subfecundity affects a woman's sense of self. I examine the role that culture plays in the content of our identities, particularly in disrupted lives. I examine the role that culture, in particular the culture within a support group, plays in the content of infertile identities. I examine how some women come to see themselves as infertile while others do not, and how the women come to think about infertility in relation to the self. I employ theories of cultural sociology and identity to provide a framework for explaining the ways in which subfecund women draw on the cultures of support groups in reconstructing their selves in the face of subfecundity.