Browsing by Subject "Sensitivity analysis"

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    Problem drinking as a risk factor for tuberculosis: a propensity score matched analysis of a national survey
    (BioMed Central Ltd, 2013) Cois, Annibale; Ehrlich, Rodney
    BACKGROUND:Epidemiological and other evidence strongly supports the hypothesis that problem drinking is causally related to the incidence of active tuberculosis and the worsening of the disease course. The presence of a large number of potential confounders, however, complicates the assessment of the actual size of this causal effect, leaving room for a substantial amount of bias. This study aims to contribute to the understanding of the role of confounding in the observed association between problem drinking and tuberculosis, assessing the effect of the adjustment for a relatively large number of potential confounders on the estimated prevalence odds ratio of tuberculosis among problem drinkers vs. moderate drinkers/abstainers in a cross-sectional, nationally representative sample of the South African adult population. METHODS: A propensity score approach was used to match each problem drinker in the sample with a subset of moderate drinkers/abstainers with similar characteristics in respect to a set of potential confounders. The prevalence odds ratio of tuberculosis between the matched groups was then calculated using conditional logistic regression. Sensitivity analyses were conducted to assess the robustness of the results in respect to misspecification of the model. RESULTS: The prevalence odds ratio of tuberculosis between problem drinkers and moderate drinkers/abstainers was 1.97 (95% CI: 1.40 to 2.77), and the result was robust with respect to the matching procedure as well as to incorrect adjustment for potential mediators and to the possible presence of unmeasured confounders. Sub-population analysis did not provide noteworthy evidence for the presence of interaction between problem drinking and the observed confounders. CONCLUSION: In a cross-sectional national survey of the adult population of a middle income country with high tuberculosis burden, problem drinking was associated with a two fold increase in the odds of past TB diagnosis after controlling for a large number of socio-economic and biological confounders. Within the limitations of a cross-sectional study design with self-reported tuberculosis status, these results adds to previous evidence of a causal link between problem drinking and tuberculosis, and suggest that the observed higher prevalence of tuberculosis among problem drinkers commonly found in population studies cannot be attributed to the confounding effect of the uneven distribution of other risk factors.
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    Why Are There Agulhas Rings?
    (1999) Pichevin, Thierry; Nof, Doron; Lutjeharms, Johann
    The recently proposed analytical theory of Nof and Pichevin describing the intimate relationship between retroflecting currents and the production of rings is examined numerically and applied to the Agulhas Current. Using a reduced-gravity 11/2-layer primitive equation model of the Bleck and Boudra type the authors show that, as the theory suggests, the generation of rings from a retroflecting current is inevitable. The generation of rings is not due to an instability associated with the breakdown of a known steady solution but rather is due to the zonal momentum flux (i.e., flow force) of the Agulhas jet that curves back on itself. Numerical experiments demonstrate that, to compensate for this flow force, several rings are produced each year. Since the slowly drifting rings need to balance the entire flow force of the retroflecting jet, their length scale is considerably larger than the Rossby radius; that is, their scale is greater than that of their classical counterparts produced by instability. Recent observations suggest a correlation between the so-called Natal Pulse and the production of Agulhas rings. As a by-product of the more general retroflection experiments, the pulse issue is also examined numerically using two different representations for the pulses. The first is a meander pulse (i.e., the pulse is similar to a meander) and the second is a transport pulse. It is shown that, in this model, there is no obvious relationship between the presence of Natal pulses and the production of rings.