Browsing by Subject "Stratification"

Now showing 1 - 3 of 3
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    The contexts of parental job loss and children's educational attainment
    (2014-12) Arbeit, Caren
    Prior research on the effects of parental job loss on children has paid little attention to the life course and contextual features of parental job loss. In my dissertation, I examine three such contexts: timing of job loss in the child's life, family socioeconomic status, the number of parental job losses and the duration of parental unemployment spells. The dissertation contains three related papers; I focus on a cohort of children in the Panel Study of Income Dynamics, to examine the relationship between these contexts of parental job loss and educational attainment at age 25. In the first paper, I draw from interdisciplinary research on parental job loss, sibling differences and life course theories. I consider whether the timing of a parent's job loss moderates the impact of the event on children's educational attainment in adulthood. The results suggest that, contrary to theory, timing is not a significant moderator. In the second paper, I examine the educational attainment of children born into socioeconomically similar families, but whom have divergent experiences related to parental job loss. I find that family SES primarily moderates the probability of experiencing a parental job loss. Finally, I focus on the impact of the number of parental job losses, the duration of parental unemployment spells and the interaction between number of job losses and the length of unemployment spells. I find that any parental job loss harms educational attainment, with a non-linear relationship between exposure to parental job loss and educational attainment at age 15. This dissertation on the timing of parental job loss, family socioeconomic background and exposure to job loss/unemployment helps create a fuller picture of the potential consequences of parental job loss on children's educational attainment.
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    The Effect of Various Stratification Lengths on Seed Sources of Ambrosia artemisiifolia Across Latitudes
    (2017-04) Kim, Joo Yoon; Moeller, David A; Gorton, Amanda J
    Seeds dormancy, a period of ceased physiological activity, is an adaptive trait promoting survival of seeds. Many plants exhibit different germination rates when seeds from various locations where seeds exposed to winter length and temperatures of their origins, are treated to equal amount of cold stratification length. This particular trait is expressed in common ragweed (Ambrosia artemisiifolia). Ragweed has high reproduction of dormant seeds that persist in the soil for many years while maintaining germination ability. Seeds are dependent on environmental conditions such as day length, temperature and humidity to break seed dormancy. That way, the dependence of seeds on environmental condition acts as a monitor system to determine the optimal germination timing and avoid improper environmental conditions. One of the most crucial environmental conditions for ragweed seeds is stratification or a period of cold temperatures, which is the first process that induces the dormancy and later promotes germination. Ragweed seeds collected from various locations corresponding to different latitudes may have different germination rates because of different temperatures and stratification from their own habitats. A study by Sorenson et al. (1990) indicated northern seeds of species western larch (Larix occidentalis Nutt.) have higher germination rates under longer stratification lengths. In the other hand, a study by Karlsson et al. (2007) indicated South African, where maintain high temperatures, seeds of species Papaver aculeatum have higher germination rates under warmer stratification. Accordingly, ragweed seeds from northern locations may require longer stratification and colder temperature to break dormancy because of its strong dormant. By contrast, seeds from southern locations may not require such long periods of stratification for germination because they lack extended winters. As difference in germination rates from different seed populations can be evident in response to various stratification lengths with fixed temperature, in this experiment I investigated the effect of different stratification lengths on germination rates of seeds from populations with different winter length.
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    Survey sampling and multiple stratifications
    (2013-09) Zimmerman, Patrick Lennon Kendall
    In survey sampling, stratied random sampling and post-stratification can increase the precision of estimation. In some cases, however, there may be multiple ways to stratify a population. We present a method, based on a non-informative Bayesian approach, that uses a finite mixture model to incorporate information from each stratification into estimation. This approach works well when the response variable is categorical or discrete,and for some non-response types of problems. We provide the theoretical basis for our method, present some simulation results, discuss various extensions, and define some software that implements the method.